Revert r211121 (and r211285), "Change libclang initialization to use std::call_once instead of"

It broke mingw builder and cygwin-clang stage2, possibly lack of tls in <mutex>.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@211593 91177308-0d34-0410-b5e6-96231b3b80d8