Add support for LLVM_USE_SANITIZER=Thread

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

Reviewers: samsonov

Reviewed By: samsonov

Subscribers: samsonov, llvm-commits

Differential Revision: http://reviews.llvm.org/D6283

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@222258 91177308-0d34-0410-b5e6-96231b3b80d8