Removing the static destructor from ManagedStatic.cpp by controlling the allocation...

Removing the static destructor from ManagedStatic.cpp by controlling the allocation and de-allocation of the mutex.

This patch adds a new llvm_call_once function which is used by the ManagedStatic implementation to safely initialize a global to avoid static construction and destruction.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@219638 91177308-0d34-0410-b5e6-96231b3b80d8