Fortify generic param default checks
* Hard-reject instead of lint-reject type param defaults in generic assoc consts (GACs) (feature: `generic_const_items`).
* In https://github.com/rust-lang/rust/pull/113522, I explicitly handled the free const item case and forgot about the assoc const one.
* This led rustc to assume the default of emitting the deny-by-default lint `invalid_type_param_default`.
* GCIs are unstable, thus we're not bound by backward compat
* Hard-reject instead of lint-reject type param defaults in foreign items.
* We already hard-reject generic params on foreign items, so this isn't a breaking change.
* There's no reason why we need to lint-reject.
* Refactor the way we determine where generic param defaults are allowed:
* Don't default to emitting lint `invalid_type_param_defaults` for nodes that aren't explicitly handled but instead panic.
* This would've caught my GAC oversight from above much earlier via fuzzing
* Prevents us from accidentally stabilizing more invalid type param defaults in the future
* Streamline the phrasing of the diagnostic
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