Sema: Fix crash during member pointer conversion involving incomplete classes

We would attempt to determine the inheritance relationship between
classes 'A' and 'B' during static_cast if we tried to convert from 'int
A::*' to 'int B::*'.  However, the question "does A derive from B" is
not meaningful when 'A' isn't defined.

Handle this case by requiring that 'A' be defined.

This fixes PR18506.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@199374 91177308-0d34-0410-b5e6-96231b3b80d8