Revert r211287, "Remove support for LLVM runtime multi-threading."

libclang still requires it on cygming, lack of incomplete <mutex>.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@211592 91177308-0d34-0410-b5e6-96231b3b80d8

lib/IR/Core.cpp

@@ -2708,10 +2708,11 @@ void LLVMDisposePassManager(LLVMPassManagerRef PM) {
 /*===-- Threading ------------------------------------------------------===*/
 
 LLVMBool LLVMStartMultithreaded() {
-  return LLVMIsMultithreaded();
+  return llvm_start_multithreaded();
 }
 
 void LLVMStopMultithreaded() {
+  llvm_stop_multithreaded();
 }
 
 LLVMBool LLVMIsMultithreaded() {