[C++20] [Modules] Skip Writing diagnostic options, header search path…

…s and pragma diagnostic mappings

It simply wastes of space and time to write diagnostic options, header
search paths and pragma diagnostic mappings for C++20 Named modules.
This patch tries to avoid the unnecessary writings.

clang/include/clang/Frontend/FrontendActions.h

@@ -151,6 +151,9 @@ class GenerateModuleInterfaceAction : public GenerateModuleAction {
 private:
   bool BeginSourceFileAction(CompilerInstance &CI) override;
 
+  std::unique_ptr<ASTConsumer> CreateASTConsumer(CompilerInstance &CI,
+                                                 StringRef InFile) override;
+
   std::unique_ptr<raw_pwrite_stream>
   CreateOutputFile(CompilerInstance &CI, StringRef InFile) override;
 };

clang/lib/Frontend/FrontendActions.cpp

@@ -258,6 +258,16 @@ bool GenerateModuleInterfaceAction::BeginSourceFileAction(
   return GenerateModuleAction::BeginSourceFileAction(CI);
 }
 
+std::unique_ptr<ASTConsumer>
+GenerateModuleInterfaceAction::CreateASTConsumer(CompilerInstance &CI,
+                                                 StringRef InFile) {
+  CI.getHeaderSearchOpts().ModulesSkipDiagnosticOptions = true;
+  CI.getHeaderSearchOpts().ModulesSkipHeaderSearchPaths = true;
+  CI.getHeaderSearchOpts().ModulesSkipPragmaDiagnosticMappings = true;
+
+  return GenerateModuleAction::CreateASTConsumer(CI, InFile);
+}
+
 std::unique_ptr<raw_pwrite_stream>
 GenerateModuleInterfaceAction::CreateOutputFile(CompilerInstance &CI,
                                                 StringRef InFile) {