Revert "Revert 220932.": "Removing the static initializer in ManagedStatic.cpp...

Revert "Revert 220932.": "Removing the static initializer in ManagedStatic.cpp by using llvm_call_once to initialize the ManagedStatic mutex"

This reverts commit r221331 and reinstate r220932 as discussed in D19271.
Original commit message was:

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.

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

From: Mehdi Amini <mehdi.amini@apple.com>
llvm-svn: 269577