tsan: Adding releaseAcquire() to ThreadClock

realeaseAcquire() is a new function added to TSan in support of the Go data-race detector.
It's semantics is:

void ThreadClock::releaseAcquire(SyncClock *sc) const {
  for (int i = 0; i < kMaxThreads; i++) {
    tmp = clock[i];
    clock[i] = max(clock[i], sc->clock[i]);
    sc->clock[i] = tmp;
  }
}

For context see: https://go-review.googlesource.com/c/go/+/220419

Reviewed-in: https://reviews.llvm.org/D76322
Author: dfava (Daniel Fava)