tsan: fix deficiency in MutexReadOrWriteUnlock

MutexUnlock uses ReleaseStore on s->clock, which is the right thing to do.
However MutexReadOrWriteUnlock for writers uses Release on s->clock.
Make MutexReadOrWriteUnlock also use ReleaseStore for consistency and performance.
Unfortunately, I don't think any test can detect this as this only potentially
affects performance.

llvm-svn: 335322