Support: Clean up TSan annotations.

Support: Clean up TSan annotations.

Remove support for Valgrind-based TSan, which hasn't been maintained for a
few years. We now use the TSan annotations only if LLVM is compiled with
-fsanitize=thread. We no longer need the weak function definitions as we
are guaranteed that our program is linked directly with the TSan runtime.

Differential Revision: http://reviews.llvm.org/D12121

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@245374 91177308-0d34-0410-b5e6-96231b3b80d8