MS ABI: Mangle null pointer-to-member-functions compatibly
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
Reviewed By: rnk
Differential Revision: http://reviews.llvm.org/D4059