[-cxx-abi microsoft] Add commentary for mangleStaticGuardVariable

We will need to do some work here if we want to play nice with MSVC2013.
Add a TODO indicating what changed and why this matters.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@197193 91177308-0d34-0410-b5e6-96231b3b80d8