[Index] Don't generate a unit file when building implicit modules

When compiling a file that requires a module to be compiled, we cloned the compiler instance for the module compile. This means that the cloned compiler instance still had the index store path and index unit output path of the original file. We were thus generating a unit file with the original file name for the compiled module. This unit file would ultimately get overwritten when the original source file was compiled, so there was no correctness issue per se but this means we were doing unnecessary work.

Disable emitting an index store for module compiles to fix this.

Author: ahoppen

clang/lib/Frontend/CompilerInstance.cpp

@@ -1357,6 +1357,8 @@ std::unique_ptr<CompilerInstance> CompilerInstance::cloneForModuleCompileImpl(
   HSOpts.ModulesHashContent = true;
   FrontendOpts.Inputs = {Input};
   FrontendOpts.MayEmitDiagnosticsAfterProcessingSourceFiles = false;
+  FrontendOpts.IndexStorePath = "";
+  FrontendOpts.IndexUnitOutputPath = "";
 
   // Don't free the remapped file buffers; they are owned by our caller.
   PPOpts.RetainRemappedFileBuffers = true;

clang/lib/Index/IndexingAction.cpp

@@ -682,12 +682,17 @@ class WrappingIndexRecordAction : public WrapperFrontendAction,
   std::unique_ptr<ASTConsumer> CreateASTConsumer(CompilerInstance &CI,
                                                  StringRef InFile) override {
     auto OtherConsumer = WrapperFrontendAction::CreateASTConsumer(CI, InFile);
-    if (!OtherConsumer)
-      return nullptr;
+
+    if (CI.getFrontendOpts().IndexStorePath.empty()) {
+      // We are not generating an index store. Nothing to do.
+      return OtherConsumer;
+    }
 
     CreatedASTConsumer = true;
     std::vector<std::unique_ptr<ASTConsumer>> Consumers;
-    Consumers.push_back(std::move(OtherConsumer));
+    if (OtherConsumer) {
+      Consumers.push_back(std::move(OtherConsumer));
+    }
     Consumers.push_back(createIndexASTConsumer(CI));
     return std::make_unique<MultiplexConsumer>(std::move(Consumers));
   }