Add shutDown method to PluginMessageHandler

This method deterministically and synchronously cleans up resources that cannot be dealt with in a deinitializer due to possible errors thrown during the clean up. Usually this includes closure of file handles, sockets, shutting down external processes and IPC resources set up for these processes, etc.

This method is required for `swift-plugin-server` to properly clean up file handles there were opened for communication with Wasm macros, as prototyped in swiftlang/swift#73031.

Author: MaxDesiatov

Sources/SwiftCompilerPluginMessageHandling/CompilerPluginMessageHandler.swift

@@ -133,6 +133,12 @@ public class CompilerPluginMessageListener<Connection: MessageConnection, Handle
 public protocol PluginMessageHandler {
   /// Handles a single message received from the plugin host.
   func handleMessage(_ message: HostToPluginMessage) -> PluginToHostMessage
+
+  /// Deterministically and synchronously cleans up resources that cannot be dealt with in
+  /// a deinitializer due to possible errors thrown during the clean up. Usually this
+  /// includes closure of file handles, sockets, shutting down external processes and IPC
+  /// resources set up for these processes, etc.
+  func shutDown() throws
 }
 
 /// A `PluginMessageHandler` that uses a `PluginProvider`.
@@ -226,6 +232,10 @@ public class PluginProviderMessageHandler<Provider: PluginProvider>: PluginMessa
       return .loadPluginLibraryResult(loaded: diags.isEmpty, diagnostics: diags)
     }
   }
+
+  /// Empty implementation for the default message handler, since all resources are automatically
+  /// cleaned up in the synthesized initializer.
+  public func shutDown() throws {}
 }
 
 @_spi(PluginMessage)