[tsan] CMake support for TSan on OS X

Hi, this patch adds a CMake flag called `COMPILER_RT_ENABLE_TSAN_OSX`, which is off by default. If enabled, the build system will be building the OS X version of the TSan runtime library (called `libclang_rt.tsan_osx_dynamic.dylib`). I'll submit patches that fix OS X build errors shortly.

This is part of an effort to port TSan to OS X, and it's one the very first steps. Don't expect TSan on OS X to actually work or pass tests at this point.

llvm-svn: 251915