MS ABI: Only class templates get complex nullptr pointer-to-member-functions

r210637 regressed CodeGenCXX/mangle-ms-templates-memptrs.cpp because it
did not believe that there is a distinction between class templates and
function templates.

Sadly, there is.  Function templates should behave in a compatible
manner with MSVC.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@210642 91177308-0d34-0410-b5e6-96231b3b80d8