[NFC][DirectoryWatchedTests] Unlocks mutexes before signaling condition variable
This should not affect actual behavior, but should pessimize the threading less
by avoiding the situation where:
* mutex is still locked
* T1 notifies on condition variable
* T2 wakes to check mutex
* T2 sees mutex is still locked
* T2 waits
* T1 unlocks mutex
* T2 tries again, acquires mutex.
Differential Revision: https://reviews.llvm.org/D65708
git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@367968
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