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.

Sponsored by <The NetBSD Foundation>

[krytarowski]

Related review: D29288

[joerg]

If we want to make this namespace aware, I wonder if we shouldn't also be explicitly and use ::llvm::once_flag etc. But in general, LGTM.

[clayborg]

LGTM, but I will let other LLVM folks give it the final approval.

[chandlerc]

The idea of this seems fine as-is, but it should be clang-formatted to fit in 80 columns.

[krytarowski]

Break line to fit the 80-cols limit.

[mehdi_amini]

LGTM.

I'd go with @joerg suggestion though (but you don't need to submit another patch, just go ahead and commit).

[krytarowski]

Use ::lvm namespace.

include/llvm/Support/Threading.h

 
 #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.
-#define LLVM_DEFINE_ONCE_FLAG(flag) static once_flag flag = Uninitialized
+#define LLVM_DEFINE_ONCE_FLAG(flag) \
+  static ::llvm::once_flag flag = ::llvm::Uninitialized
 
 #endif
 
   /// \brief Execute the function specified as a parameter once.
   ///
   /// Typical usage:
   /// \code
   ///   void foo() {...};