MS ABI: Disambiguate the manglings for global guard variables

If a guard variable will be created for an entity at global scope,
then we cannot rely on the scope depth to disambiguate names for us.

Instead, mangle the entire variable into the guard to ensure it's uniqueness.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@203151 91177308-0d34-0410-b5e6-96231b3b80d8