For Clang's thread safety analysis (-Wthread-safety) to pass without warnings,
we need to annotate WrappedMutex to indicate that Lock() acquires the lock and
Unlock() releases it. This CL adds the annotations and guards them with an
ifdef to make sure they're a no-op on compilers other than Clang.

This is a cherry-pick of a change I already made to the Google-internal
codebase.