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

Summary:
This patch adds an llvm_call_once which is a wrapper around std::call_once on platforms where it is available and devoid of bugs. The patch also migrates the ManagedStatic mutex to be allocated using llvm_call_once.

These changes are philosophically equivalent to the changes added in r219638, which were reverted due to a hang on Win32 which was the result of a bug in the Windows implementation of std::call_once.

Reviewers: aaron.ballman, chapuni, chandlerc, rnk

Reviewed By: rnk

Subscribers: majnemer, llvm-commits

Differential Revision: http://reviews.llvm.org/D5922