Rollup merge of #108364 - Nilstrieb:validity-checks-refactor, r=compiler-errors

Unify validity checks into a single query

Previously, there were two queries to check whether a type allows the 0x01 or zeroed bitpattern.

I am planning on adding a further initness to check in #100423, truly uninit for MaybeUninit, which would make this three queries. This seems overkill for such a small feature, so this PR unifies them into one.

I am not entirely happy with the naming and key type and open for improvements.

r? oli-obk

Author: matthiaskrgr