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