[libcxx] Enable building and testing of libcxx with ThreadSanitizer on OS X

[kubamracek]

Summary

This patch enables building and testing libcxx under ThreadSanitizer on OS X. CMake builds that have -DLLVM_USE_SANITIZER=Thread will automatically build libcxx with -fsanitize=thread and testing via lit then runs under TSan. There was one more typo in a python config script, but after that is fixed I verified that the TSan build works.

This diff also adds a regression test for the bug (data race) fixed in https://reviews.llvm.org/D24028. If you revert the fix, TSan will report a data race in call_once. If approved, I’ll commit the test separately.

[kubamracek]

ping

libcxx/trunk/lib/CMakeLists.txt

 add_library_flags("${LIBCXX_CXX_ABI_LIBRARY}")
 add_library_flags_if(LIBCXX_ENABLE_STATIC_ABI_LIBRARY "-Wl,-Bdynamic" "-Wl,--no-whole-archive")
 
 if (APPLE AND LLVM_USE_SANITIZER)
   if ("${LLVM_USE_SANITIZER}" STREQUAL "Address")
     set(LIBFILE "libclang_rt.asan_osx_dynamic.dylib")
   elseif("${LLVM_USE_SANITIZER}" STREQUAL "Undefined")
     set(LIBFILE "libclang_rt.ubsan_osx_dynamic.dylib")
+  elseif("${LLVM_USE_SANITIZER}" STREQUAL "Thread")
+    set(LIBFILE "libclang_rt.tsan_osx_dynamic.dylib")
   else()
     message(WARNING "LLVM_USE_SANITIZER=${LLVM_USE_SANITIZER} is not supported on OS X")
   endif()
   if (LIBFILE)
     execute_process(COMMAND ${CMAKE_CXX_COMPILER} -print-file-name=lib OUTPUT_VARIABLE LIBDIR RESULT_VARIABLE Result)
     if (NOT ${Result} EQUAL "0")
       message(FATAL "Failed to find library resource directory")
     endif()