Clang side changes for removal of support for runtime multi-threading.

[zturner]

Summary

Clang side changes for removal of support for runtime multi-threading.

[rnk]

lgtm

Will commit in a moment.

tools/libclang/CIndex.cpp

2566

Unrelated issue: I wonder if this should call fflush(stderr).

[zturner]

Closed by commit rL210601 (authored by @zturner).

tools/libclang/CIndex.cpp

buildPieces(unsigned NameFlags, bool IsMemberRefExpr,
   return Pieces;  
 }
 }
 
 //===----------------------------------------------------------------------===//
 // Misc. API hooks.
 //===----------------------------------------------------------------------===//               
 
-static llvm::sys::Mutex EnableMultithreadingMutex;
-static bool EnabledMultithreading;
+static llvm::sys::Mutex LoggingMutex;
+static std::once_flag InstalledErrorHandlerFlag;
 
 static void fatal_error_handler(void *user_data, const std::string& reason,
                                 bool gen_crash_diag) {
   // Write the result out to stderr avoiding errs() because raw_ostreams can
   // call report_fatal_error.
   fprintf(stderr, "LIBCLANG FATAL ERROR: %s\n", reason.c_str());
   ::abort();
 }
 
 extern "C" {
 CXIndex clang_createIndex(int excludeDeclarationsFromPCH,
                           int displayDiagnostics) {
   // We use crash recovery to make some of our APIs more reliable, implicitly
   // enable it.
   if (!getenv("LIBCLANG_DISABLE_CRASH_RECOVERY"))
     llvm::CrashRecoveryContext::Enable();
 
-  // Enable support for multithreading in LLVM.
-  {
-    llvm::sys::ScopedLock L(EnableMultithreadingMutex);
-    if (!EnabledMultithreading) {
-      llvm::install_fatal_error_handler(fatal_error_handler, nullptr);
-      llvm::llvm_start_multithreaded();
-      EnabledMultithreading = true;
-    }
-  }
+  std::call_once(InstalledErrorHandlerFlag,
+      []() {llvm::install_fatal_error_handler(fatal_error_handler, nullptr);});
 
   CIndexer *CIdxr = new CIndexer();
   if (excludeDeclarationsFromPCH)
     CIdxr->setOnlyLocalDecls();
   if (displayDiagnostics)
     CIdxr->setDisplayDiagnostics();
 
   if (getenv("LIBCLANG_BGPRIO_INDEX"))

 Logger &cxindex::Logger::operator<<(const llvm::format_object_base &Fmt) {
   LogOS << Fmt;
   return *this;
 }
 
 cxindex::Logger::~Logger() {
   LogOS.flush();
 
-  llvm::sys::ScopedLock L(EnableMultithreadingMutex);
+  llvm::sys::ScopedLock L(LoggingMutex);
 
   static llvm::TimeRecord sBeginTR = llvm::TimeRecord::getCurrentTime();
 
   raw_ostream &OS = llvm::errs();
   OS << "[libclang:" << Name << ':';
 
   // FIXME: Portability.
 #ifdef __APPLE__