This fixes libclang to cope with the now compile-time multithreaded
selection re-enabled in r211900 in LLVM.

The approach (unlike r211121) doesn't rely on std::mutex or
std::call_once to avoid breaknig cygwin bots.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@211901 91177308-0d34-0410-b5e6-96231b3b80d8