[MS ABI] Select a pointer to member representation more often

Given a reference to a pointer to member whose class's inheritance model
is unspecified, make sure we come up with an inheritance model in
plausible places.  One place we were missing involved LValue to RValue
conversion, another involved unary type traits.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@247248 91177308-0d34-0410-b5e6-96231b3b80d8