Add support for LLVM_USE_SANITIZER=Thread

[EricWF]

Summary

This will help in testing libc++ and libc++abi with tsan.

[samsonov]

Looks reasonable. Have you tested LLVM test suite with this option enabled? Does it pass?

[EricWF]

Thanks for the review. I just did a run and saw about 4 failures in the LLVM test suite. The build was clean though.

[EricWF]

Closed by commit rL222258 (authored by @EricWF).

cmake/modules/HandleLLVMOptions.cmake

       append_common_sanitizer_flags()
       append("-fsanitize=undefined -fno-sanitize=vptr,function -fno-sanitize-recover"
               CMAKE_C_FLAGS CMAKE_CXX_FLAGS)
+    elseif (LLVM_USE_SANITIZER STREQUAL "Thread")
+      append_common_sanitizer_flags()
+      append("-fsanitize=thread" CMAKE_C_FLAGS CMAKE_CXX_FLAGS)
     else()
       message(WARNING "Unsupported value of LLVM_USE_SANITIZER: ${LLVM_USE_SANITIZER}")
     endif()