[tsan] Add Clang frontend support for TSan on OS X

[kubamracek]

Summary

We're currently in process of porting TSan to OS X, and quite a few of the initial support in the runtime library has already landed in trunk. This patch actually enables "-fsanitize=thread" in the frontend.

lib/Driver/ToolChains.cpp

if (const Arg *A = Args.getLastArg(options::OPT_static_libgcc)) {
     return;
   }
 
   const SanitizerArgs &Sanitize = getSanitizerArgs();
   if (Sanitize.needsAsanRt())
     AddLinkSanitizerLibArgs(Args, CmdArgs, "asan");
   if (Sanitize.needsUbsanRt())
     AddLinkSanitizerLibArgs(Args, CmdArgs, "ubsan");
+  if (Sanitize.needsTsanRt())
+    AddLinkSanitizerLibArgs(Args, CmdArgs, "tsan");
 
   // Otherwise link libSystem, then the dynamic runtime library, and finally any
   // target specific static runtime library.
   CmdArgs.push_back("-lSystem");
 
   // Select the dynamic runtime library and the target specific static library.
   if (isTargetWatchOSBased()) {
     // We currently always need a static runtime library for watchOS.

 SanitizerMask Darwin::getSupportedSanitizers() const {
   SanitizerMask Res = ToolChain::getSupportedSanitizers();
   if (isTargetMacOS() || isTargetIOSSimulator())
     Res |= SanitizerKind::Address;
   if (isTargetMacOS()) {
     if (!isMacosxVersionLT(10, 9))
       Res |= SanitizerKind::Vptr;
     Res |= SanitizerKind::SafeStack;
+    Res |= SanitizerKind::Thread;
   }
   return Res;
 }
 
 /// Generic_GCC - A tool chain using the 'gcc' command to perform
 /// all subcommands; this relies on gcc translating the majority of
 /// command line options.