[clang] Add getCommentHandler to PreambleCallbacks

[kadircet]

Summary

Enables users to add comment handlers to preprocessor when building
preambles.

[ilya-biryukov]

LGTM, just a few nits

include/clang/Frontend/PrecompiledPreamble.h

286

NIT: this function does not add anything by itself, maybe rephrase to avoid confusion, e.g. The returned CommentHandler will be added to the preprocessor if not null.

NIT: Use Preprocessor or preprocessor. It seems to be always spelled as a single word.

[kadircet]

• Update comment

include/clang/Frontend/PrecompiledPreamble.h

public:
   virtual void AfterPCHEmitted(ASTWriter &Writer);
   /// Called for each TopLevelDecl.
   /// NOTE: To allow more flexibility a custom ASTConsumer could probably be
   /// used instead, but having only this method allows a simpler API.
   virtual void HandleTopLevelDecl(DeclGroupRef DG);
   /// Creates wrapper class for PPCallbacks so we can also process information
   /// about includes that are inside of a preamble
   virtual std::unique_ptr<PPCallbacks> createPPCallbacks();
+  /// The returned CommentHandler will be added to the preprocessor if not null.
+  virtual CommentHandler *getCommentHandler();
 };
 
 enum class BuildPreambleError {
   CouldntCreateTempFile = 1,
   CouldntCreateTargetInfo,
   BeginSourceFileFailed,
   CouldntEmitPCH
 };

lib/Frontend/PrecompiledPreamble.cpp

 #include "clang/Frontend/PrecompiledPreamble.h"
 #include "clang/AST/DeclObjC.h"
 #include "clang/Basic/TargetInfo.h"
 #include "clang/Frontend/CompilerInstance.h"
 #include "clang/Frontend/CompilerInvocation.h"
 #include "clang/Frontend/FrontendActions.h"
 #include "clang/Frontend/FrontendOptions.h"
 #include "clang/Lex/Lexer.h"
+#include "clang/Lex/Preprocessor.h"
 #include "clang/Lex/PreprocessorOptions.h"
 #include "clang/Serialization/ASTWriter.h"
 #include "llvm/ADT/StringExtras.h"
 #include "llvm/ADT/StringSet.h"
 #include "llvm/Config/llvm-config.h"
 #include "llvm/Support/CrashRecoveryContext.h"
 #include "llvm/Support/FileSystem.h"
 #include "llvm/Support/Mutex.h"
llvm::ErrorOr<PrecompiledPreamble> PrecompiledPreamble::Build(
   Callbacks.BeforeExecute(*Clang);
   if (!Act->BeginSourceFile(*Clang.get(), Clang->getFrontendOpts().Inputs[0]))
     return BuildPreambleError::BeginSourceFileFailed;
 
   std::unique_ptr<PPCallbacks> DelegatedPPCallbacks =
       Callbacks.createPPCallbacks();
   if (DelegatedPPCallbacks)
     Clang->getPreprocessor().addPPCallbacks(std::move(DelegatedPPCallbacks));
+  if (auto CommentHandler = Callbacks.getCommentHandler())
+    Clang->getPreprocessor().addCommentHandler(CommentHandler);
 
   Act->Execute();
 
   // Run the callbacks.
   Callbacks.AfterExecute(*Clang);
 
   Act->EndSourceFile();
 

 
 void PreambleCallbacks::BeforeExecute(CompilerInstance &CI) {}
 void PreambleCallbacks::AfterExecute(CompilerInstance &CI) {}
 void PreambleCallbacks::AfterPCHEmitted(ASTWriter &Writer) {}
 void PreambleCallbacks::HandleTopLevelDecl(DeclGroupRef DG) {}
 std::unique_ptr<PPCallbacks> PreambleCallbacks::createPPCallbacks() {
   return nullptr;
 }
+CommentHandler *PreambleCallbacks::getCommentHandler() { return nullptr; }
 
 static llvm::ManagedStatic<BuildPreambleErrorCategory> BuildPreambleErrCategory;
 
 std::error_code clang::make_error_code(BuildPreambleError Error) {
   return std::error_code(static_cast<int>(Error), *BuildPreambleErrCategory);
 }
 
 const char *BuildPreambleErrorCategory::name() const noexcept {