tsan: fix handling of condition variable destruction

POSIX states that "It shall be safe to destroy an initialized condition
variable upon which no threads are currently blocked", and later clarifies
"A condition variable can be destroyed immediately after all the threads
that are blocked on it are awakened) (in examples section). Tsan reported
such destruction as a data race.
Fixes https://llvm.org/bugs/show_bug.cgi?id=23616
Reviewed in http://reviews.llvm.org/D10693

llvm-svn: 241082