libclang: make darwin pthread hacks conditional on LLVM_ENABLE_THREADS

Although these aren't strictly related to LLVM's core threading, it's
reasonable to avoid pthread usage in clang when building with
LLVM_ENABLE_THREADS disabled.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@212395 91177308-0d34-0410-b5e6-96231b3b80d8