Tracking issue for layout sanity checks

[RalfJung]

This is not tracking a user-visible feature, but it's still a tracking issue of sorts...

In #96872 I have started implemented some checks we run on all computed layouts (when debug assertions are enabled), because it turns out we do have invariants on those layouts and they do get violated. There is still a lot more that can be checked though.

Implementation history:

• make sure ScalarPair enums have ScalarPair variants; add some layout sanity checks #96872
• assert Scalar sanity #98968
• More layout sanity checks #99983

Cc @eddyb

[RalfJung]

In #99983 (comment) @eddyb suggested a way to check Even More Things (TM), including a lot of code already almost implementing that.