Auto merge of #115290 - compiler-errors:ctor-unsafe, r=cjgillot

`rustc_layout_scalar_valid_range` makes ctors unsafe

We already validate this when we use the ctor in a call, e.g. `Variant(1)`, but not if we use the ctor as a fn ptr, e.g. `.map(Variant)`. The easiest way to fix the latter is (afaict) is by marking the ctor as unsafe itself.

Fixes #115284

Author: bors