Win64: Use ConvertType instead of checking the MS inheritance

dependent-type-member-pointer.cpp is failing on a win64 bot because
-fms-extensions is not enabled.  Use ConvertType rather than relying on
the inheritance attributes.  It's less code, but probably slower.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@207819 91177308-0d34-0410-b5e6-96231b3b80d8