[clang] Unify Sema and CodeGen implementation of isFlexibleArrayMemberExpr

Turn it into a single Expr::isFlexibleArrayMemberLike method, as discussed in

        https://discourse.llvm.org/t/rfc-harmonize-flexible-array-members-handling

Keep different behavior with respect to macro / template substitution, and
harmonize sharp edges: ObjC interface now behave as C struct wrt. FAM and
-fstrict-flex-arrays.

This does not impact __builtin_object_size interactions with FAM.

Differential Revision: https://reviews.llvm.org/D134791