stage2: fix -Domit-stage2 regression

This flag is used when building stage1 to omit the stage2 backends from
the compiler to save memory on the CI server. It regressed with the
merging of e8813b2 because Value
functions started calling into Sema functions.

The end goal for this build option is to eliminate it.

Author: andrewrk

src/Compilation.zig

@@ -2042,7 +2042,8 @@ pub fn update(comp: *Compilation) !void {
         comp.c_object_work_queue.writeItemAssumeCapacity(key);
     }
 
-    const use_stage1 = build_options.is_stage1 and comp.bin_file.options.use_stage1;
+    const use_stage1 = build_options.omit_stage2 or
+        (build_options.is_stage1 and comp.bin_file.options.use_stage1);
     if (comp.bin_file.options.module) |module| {
         module.compile_log_text.shrinkAndFree(module.gpa, 0);
         module.generation += 1;
@@ -2198,7 +2199,8 @@ fn flush(comp: *Compilation) !void {
     try comp.bin_file.flush(comp); // This is needed before reading the error flags.
     comp.link_error_flags = comp.bin_file.errorFlags();
 
-    const use_stage1 = build_options.is_stage1 and comp.bin_file.options.use_stage1;
+    const use_stage1 = build_options.omit_stage2 or
+        (build_options.is_stage1 and comp.bin_file.options.use_stage1);
     if (!use_stage1) {
         if (comp.bin_file.options.module) |module| {
             try link.File.C.flushEmitH(module);

src/Sema.zig

@@ -89,6 +89,7 @@ const RangeSet = @import("RangeSet.zig");
 const target_util = @import("target.zig");
 const Package = @import("Package.zig");
 const crash_report = @import("crash_report.zig");
+const build_options = @import("build_options");
 
 pub const InstMap = std.AutoHashMapUnmanaged(Zir.Inst.Index, Air.Inst.Ref);
 
@@ -20808,6 +20809,9 @@ pub fn resolveTypeLayout(
     src: LazySrcLoc,
     ty: Type,
 ) CompileError!void {
+    if (build_options.omit_stage2)
+        @panic("sadly stage2 is omitted from this build to save memory on the CI server");
+
     switch (ty.zigTypeTag()) {
         .Struct => return sema.resolveStructLayout(block, src, ty),
         .Union => return sema.resolveUnionLayout(block, src, ty),
@@ -20974,6 +20978,8 @@ fn resolveUnionFully(
 }
 
 pub fn resolveTypeFields(sema: *Sema, block: *Block, src: LazySrcLoc, ty: Type) CompileError!Type {
+    if (build_options.omit_stage2)
+        @panic("sadly stage2 is omitted from this build to save memory on the CI server");
     switch (ty.tag()) {
         .@"struct" => {
             const struct_obj = ty.castTag(.@"struct").?.data;
@@ -22256,6 +22262,8 @@ fn typePtrOrOptionalPtrTy(
 /// TODO merge these implementations together with the "advanced"/sema_kit pattern seen
 /// elsewhere in value.zig
 pub fn typeRequiresComptime(sema: *Sema, block: *Block, src: LazySrcLoc, ty: Type) CompileError!bool {
+    if (build_options.omit_stage2)
+        @panic("sadly stage2 is omitted from this build to save memory on the CI server");
     return switch (ty.tag()) {
         .u1,
         .u8,