[clang][ExprConstant] fix __builtin_object_size for flexible array members

As reported by @kees, GCC treats __builtin_object_size of structures
containing flexible array members (aka arrays with incomplete type) not
just as the sizeof the underlying type, but additionally the size of the
members in a designated initializer list.

Fixes: https://github.com/llvm/llvm-project/issues/62789

Reviewed By: erichkeane

Differential Revision: https://reviews.llvm.org/D150892