[clang][Interp] Member Pointers (#91303)

This adds a `MemberPointer` class along with a `PT_MemberPtr` primitive
type.

A `MemberPointer` has a `Pointer` Base as well as a `Decl*` (could be
`ValueDecl*`?) decl it points to.
For the actual logic, this mainly changes the way we handle `PtrMemOp`s
in `VisitBinaryOperator`.