Add warning for badly staged types.

Resolves #23829.

The stage violation results in out-of-bound names in splices.
Technically this is an error, but someone might rely on this!?

Internal changes:
- we now track stages for TyVars.
- thLevel (RunSplice _) = 0, instead of panic, as reifyInstances does
  in fact rename its argument type, and it can contain variables.