Auto merge of #13866 - Nilstrieb:rustc_safe_intrinsic, r=Veykril

Use `rustc_safe_intrinsic` attribute to check for intrinsic safety

Instead of maintaining a list that is poorly kept in sync we can just use the attribute.

This will make new RA versions unusable with old toolchains that don't have the attribute yet. Should we keep maintaining the list as a fallback or just don't care?