add a custom diagnostic for type mismatches of constants

[oli-obk]

Diagnostics changes here are meh, but I think we should fix those on the diagnostics side. Maybe we can relate a different TypeError kind for const kind mismatches that signal something is wrong in the source.

Originally posted by compiler-errors in #125451 (review)

The new diagnostic should specifically talk about the fact that the constants have different types

[oli-obk]

resolved by #125671 and follow-ups