[ms-cxxabi] Correctly compute the size of member pointers

Summary:
This also relaxes the requirement on Windows that the member pointer
class type be a complete type (http://llvm.org/PR12070).  We still ask
for a complete type to instantiate any templates (MSVC does this), but
if that fails we continue as normal, relying on any inheritance
attributes on the declaration.

Reviewers: rjmccall

CC: triton, timurrrr, cfe-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D568

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@178283 91177308-0d34-0410-b5e6-96231b3b80d8