tsan: fix a bug in trace part switching

Callers of TraceSwitchPart expect that TraceAcquire will always succeed
after the call. It's possible that TryTraceFunc/TraceMutexLock in TraceSwitchPart
that restore the current stack/mutexset filled the trace part exactly up
to the TracePart::kAlignment gap and the next TraceAcquire won't succeed.
Skip the alignment gap after writing initial stack/mutexset to avoid that.

Reviewed By: melver

Differential Revision: https://reviews.llvm.org/D129777