[MS] Fix constexpr data member pointer conversions

Constexpr data member conversions work by starting with the class that
originally introduced the field, and converting from there to the type
that the user desires. Before this change, Clang was using the
inheritance model from the final destination class type instead of the
model from the class that originally introduced the field. To fix this,
find the relevant FieldDecl and take its parent class instead of using
the member pointer type the user provided.

Indirect field decls require some special handling to find the parent
class.

Fixes PR43803