[MS ABI, DebugInfo] Omit the size for model-less pointers-to-members

The representation of a pointer-to-member in the MS ABI is governed by
the layout of the relevant class or if a model has been explicitly
specified.  If no model is specified, then an appropriate
"worst-case-scenario" model is implicitly chosen if, and only, if the
pointer-to-member type's representation was needed.

Debug info cannot force a pointer-to-member type to have a
representation so do not try to query the size of such a type unless we
know it is safe to do so.

llvm-svn: 238259