Support: add llvm::unique_lock
Based on the STL class of the same name, it guards a mutex
while also allowing it to be unlocked conditionally before
destruction.
This eliminates the last naked usages of mutexes in LLVM and
clang.
It also uncovered and fixed a bug in callExternalFunction()
when compiled without USE_LIBFFI, where the mutex would never
be unlocked if the end of the function was reached.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@216338
91177308-0d34-0410-b5e6-
96231b3b80d8
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