Constant validation is not cached

[oli-obk]

Constant validation is not part of the query that evaluates constants, because it would cause cycle errors in case a static directly or transitively contained a pointer back to itself.

One possible solution is to wrap another query around the const_eval query, not exposing the const_eval query to regular callers (similar to is_freeze_raw) and having a wrapper query around it that does the validation and thus also caches the validation.

[RalfJung]

The same mechanism is also needed for generic constants to get any validation at all.

Also, when we are adding "wrapper queries", it might be a good idea to have separate queries for const and static. The current situation there (about who reports which errors when, and about whether the result must be ByRef or must be normalized to use Scalar where possible) is rather messy.