MS ABI: Mangle null pointer-to-member-functions compatibly

Summary:
Previously, we would mangle nullptr pointer-to-member-functions in class
templates with a mangling we invented because contemporary versions of
MSVC would crash when trying to compile such code.

However, VS "14" can successfully compile these sorts of template
instantiations.  This commit updates our mangling to be compatible with
theirs.

Reviewers: rnk

Reviewed By: rnk

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D4059