Remove circular header reference in Threading.h/Mutex.h

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@211290 91177308-0d34-0410-b5e6-96231b3b80d8
Remove use of removed function, llvm_stop_multithreading

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@211291 91177308-0d34-0410-b5e6-96231b3b80d8