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

llvm-svn: 245374