Add support for tsan annotations (thread sanitizer, a valgrind-based tool).

Add support for tsan annotations (thread sanitizer, a valgrind-based tool).
These annotations are disabled entirely when either ENABLE_THREADS is off, or
building a release build. When enabled, they add calls to functions with no
statements to ManagedStatic's getters.

Use these annotations to inform tsan that the race used inside ManagedStatic
initialization is actually benign. Thanks to Kostya Serebryany for helping
write this patch!

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@144567 91177308-0d34-0410-b5e6-96231b3b80d8