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