[Explicit Module Builds][Target Variant] Refactor build planning to allow for a second, separte emit-module task for the target variant

This change allows builds that specify a variant triple and which must emit a target variant module to do so from within a single `swiftc` invocation when using Explicit Module Builds.

This is an extensive refactor of the build planning procedure with respect to Explicit Module Builds:
- Remove 'ExplicitDependencyBuildPlanner' global state from the 'Driver' instance
- Instantiate a 'ExplicitDependencyBuildPlanner' instance at the beginning if build planning and propagate/thread this instance to all the required compilation task creation routines, all the way to 'addCommonFrontendOptions'
- For '-target-variant' builds which emit a variant triple module, instantiate a whole separate explicit build planner, including performing a secondary dependency scan using the variant triple. Using this information:
  - Schedule variant triple module pre-compile dependency tasks
  - Schedule a variant triple PCH compilation task, if necessary
  - Schedule the variant emit-module task
  - Schedule the variant textual module verification task, if necessary

Author: artemcm

Sources/SwiftDriver/Driver/Driver.swift

@@ -341,38 +341,15 @@ public struct Driver {
   /// Original ObjC Header passed from command-line
   let originalObjCHeaderFile: VirtualPath.Handle?
 
-
   /// Enable bridging header chaining.
   let bridgingHeaderChaining: Bool
 
-  /// The path to the imported Objective-C header.
-  lazy var importedObjCHeader: VirtualPath.Handle? = {
-    assert(explicitDependencyBuildPlanner != nil ||
-           !parsedOptions.hasArgument(.driverExplicitModuleBuild) ||
-           !inputFiles.contains { $0.type == .swift },
-           "should not be queried before scanning")
-    let chainedBridgingHeader = try? explicitDependencyBuildPlanner?.getChainedBridgingHeaderFile()
-    return try? computeImportedObjCHeader(&parsedOptions, compilerMode: compilerMode,
-                                          chainedBridgingHeader: chainedBridgingHeader) ?? originalObjCHeaderFile
-  }()
-
   /// The directory to emit PCH file.
   lazy var bridgingPrecompiledHeaderOutputDir: VirtualPath? = {
     return try? computePrecompiledBridgingHeaderDir(&parsedOptions,
                                                     compilerMode: compilerMode)
   }()
 
-  /// The path to the pch for the imported Objective-C header.
-  lazy var bridgingPrecompiledHeader: VirtualPath.Handle? = {
-    let contextHash = try? explicitDependencyBuildPlanner?.getMainModuleContextHash()
-    return computeBridgingPrecompiledHeader(&parsedOptions,
-                                            compilerMode: compilerMode,
-                                            importedObjCHeader: importedObjCHeader,
-                                            outputFileMap: outputFileMap,
-                                            outputDirectory: bridgingPrecompiledHeaderOutputDir,
-                                            contextHash: contextHash)
-  }()
-
   /// Path to the dependencies file.
   let dependenciesFilePath: VirtualPath.Handle?
 
@@ -621,14 +598,6 @@ public struct Driver {
   /// The mode the API digester should run in.
   let digesterMode: DigesterMode
 
-  // FIXME: We should soon be able to remove this from being in the Driver's state.
-  // Its only remaining use outside of actual dependency build planning is in
-  // command-line input option generation for the explicit main module compile job.
-  /// Planner for constructing module build jobs using Explicit Module Builds.
-  /// Constructed during the planning phase only when all module dependencies will be prebuilt and treated
-  /// as explicit inputs by the various compilation jobs.
-  @_spi(Testing) public var explicitDependencyBuildPlanner: ExplicitDependencyBuildPlanner? = nil
-
   /// A reference to the instance of libSwiftScan which is shared with the driver's
   /// `InterModuleDependencyOracle`, but also used for non-scanning tasks, such as target info
   /// and supported compiler feature queries
@@ -1357,9 +1326,9 @@ public struct Driver {
   }
 
   public mutating func planBuild() throws -> [Job] {
-    let (jobs, incrementalCompilationState, intermoduleDependencyGraph) = try planPossiblyIncrementalBuild()
+    let (jobs, incrementalCompilationState, explicitModulePlanner) = try planPossiblyIncrementalBuild()
     self.incrementalCompilationState = incrementalCompilationState
-    self.intermoduleDependencyGraph = intermoduleDependencyGraph
+    self.intermoduleDependencyGraph = explicitModulePlanner?.dependencyGraph
     return jobs
   }
 }
@@ -1748,11 +1717,9 @@ extension Diagnostic.Message {
 }
 
 extension Driver {
-  func explainModuleDependency(_ explainModuleName: String, allPaths: Bool) throws {
-    guard let dependencyPlanner = explicitDependencyBuildPlanner else {
-      fatalError("Cannot explain dependency without Explicit Build Planner")
-    }
-    guard let dependencyPaths = try dependencyPlanner.explainDependency(explainModuleName, allPaths: allPaths) else {
+  func explainModuleDependency(_ explainModuleName: String, allPaths: Bool,
+                               moduleDependencyGraph: InterModuleDependencyGraph) throws {
+    guard let dependencyPaths = try moduleDependencyGraph.explainDependency(explainModuleName, allPaths: allPaths) else {
       diagnosticEngine.emit(.remark("No such module dependency found: '\(explainModuleName)'"))
       return
     }
@@ -1824,13 +1791,6 @@ extension Driver {
       return
     }
 
-    // If we're only supposed to explain a dependency on a given module, do so now.
-    if let explainModuleName = parsedOptions.getLastArgument(.explainModuleDependencyDetailed) {
-      try explainModuleDependency(explainModuleName.asSingle, allPaths: true)
-    } else if let explainModuleNameDetailed = parsedOptions.getLastArgument(.explainModuleDependency) {
-      try explainModuleDependency(explainModuleNameDetailed.asSingle, allPaths: false)
-    }
-
     if parsedOptions.contains(.driverPrintOutputFileMap) {
       if let outputFileMap = self.outputFileMap {
         stderrStream.send(outputFileMap.description)
@@ -2023,7 +1983,7 @@ extension Driver {
 
     // Put bridging header as first input if we have it
     let allInputs: [TypedVirtualPath]
-    if let objcHeader = importedObjCHeader, bridgingPrecompiledHeader != nil {
+    if let objcHeader = originalObjCHeaderFile {
       allInputs = [TypedVirtualPath(file: objcHeader, type: .objcHeader)] + inputFiles
     } else {
       allInputs = inputFiles
@@ -2048,10 +2008,7 @@ extension Driver {
       // All input action IDs for this action.
       var inputIds = [UInt]()
 
-      var jobInputs = job.primaryInputs.isEmpty ? job.inputs : job.primaryInputs
-      if let pchPath = bridgingPrecompiledHeader, job.kind == .compile {
-        jobInputs.append(TypedVirtualPath(file: pchPath, type: .pch))
-      }
+      let jobInputs = job.primaryInputs.isEmpty ? job.inputs : job.primaryInputs
       // Collect input job IDs.
       for input in jobInputs {
         if let id = inputIdMap[input] {