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

[Chris Bieneman]

While I recognize that Chandler's point is probably right that we should use the Windows API (caveat that we're ok with dropping running on XP as Reid pointed out).

Would it be reasonable for me to land the hand-rolled double checked solution instead, and have someone else (preferably someone more skilled with Windows) implement the Windows API based solution later?

http://reviews.llvm.org/D5922