Make llvm::call_once more convenient to reuse out of LLVM

[krytarowski]

Summary

Currently users need to set call using namespace llvm;, with this change it's no longer needed.

This patch is required for systems defining LLVM_THREADING_USE_STD_CALL_ONCE.

Sponsored by <The NetBSD Foundation>

[krytarowski]

I'm going to propose a reworked interface. Closer in interaction to std::call_once.

include/llvm/Support/Threading.h

void llvm_execute_on_thread(void (*UserFn)(void*), void *UserData,
                               unsigned RequestedStackSize = 0);
 
 #if LLVM_THREADING_USE_STD_CALL_ONCE
 
   typedef std::once_flag once_flag;
 
   /// This macro is the only way you should define your once flag for LLVM's
   /// call_once.
-#define LLVM_DEFINE_ONCE_FLAG(flag) static once_flag flag
+#define LLVM_DEFINE_ONCE_FLAG(flag) static ::llvm::once_flag flag
 
 #else
 
   enum InitStatus { Uninitialized = 0, Wait = 1, Done = 2 };
   typedef volatile sys::cas_flag once_flag;
 
   /// This macro is the only way you should define your once flag for LLVM's
   /// call_once.