Constexpr evaluator should treat [[gnu::weak]] member pointer comparisons as evaluation failure

This patch fixes the wrong signal from the constexpr evaluator that
[[gnu::weak]] member pointer comparison is valid, while it is emitting
notes on them.

I found a crashing case fixed by this change and added it as a test
case: https://godbolt.org/z/8391fGjGn

I noticed this while I was working on D146358.

Differential Revision: https://reviews.llvm.org/D148419