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

clang/lib/Frontend/CompilerInstance.cpp

@@ -1357,6 +1357,15 @@ std::unique_ptr<CompilerInstance> CompilerInstance::cloneForModuleCompileImpl(
   HSOpts.ModulesHashContent = true;
   FrontendOpts.Inputs = {Input};
   FrontendOpts.MayEmitDiagnosticsAfterProcessingSourceFiles = false;
+  // Clear `IndexUnitOutputPath`. Otherwise the unit for the pcm will be named
+  // after the primary source file, and will be overwritten when that file's
+  // unit is finally written.
+  FrontendOpts.IndexUnitOutputPath = "";
+  if (FrontendOpts.IndexIgnorePcms) {
+    // If we shouldn't index pcms, disable indexing by clearing the index store
+    // path.
+    FrontendOpts.IndexStorePath = "";
+  }
 
   // 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));
   }