[MS ABI] Don't crash on references to pointers to members in args

We know that a reference can always be dereferenced.  However, we don't
always know the number of bytes if the reference's pointee type is
incomplete.  This case was correctly handled but we didn't consider the
case where the type is complete but we cannot calculate its size for ABI
specific reasons.  In this specific case, a member pointer's size is
available only under certain conditions.

This fixes PR24703.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@247188 91177308-0d34-0410-b5e6-96231b3b80d8