[PATCH] Removing the static initializer in ManagedStatic.cpp by using llvm_call_once to initialize the ManagedStatic mutex.

[Chris Bieneman]

Ugh... So, std::atomic is off the table.

I think we can do this instead:

```
void llvm::call_once(once_flag &Initialized, void (*fptr)(void)) {
  while (flag != Done) {
    if (flag == Wait) {
      ::Sleep(1);
      continue;
    }

    sys::cas_flag old_val = sys::CompareAndSwap(&flag, Wait, Uninitialized);
    if (old_val == Uninitialized) {
      fptr();
      sys::MemoryFence();
      flag = Done;
    }
  }
  sys::MemoryFence();
}

```
It ends up being a bit conservative on MemFences on x86, but it should be safe, and since once_flag is just an unsigned, it has a trivial static initializer and should avoid the issue that std::atomic has.

http://reviews.llvm.org/D5922