Fix #16391 by using occCheckExpand in TcValidity

The type-variables-escaping-their-scope-via-kinds check in TcValidity was failing to properly expand type synonyms, which led to #16391 (closed). This is easily fixed by using occCheckExpand before performing the validity check.

Along the way, I refactored this check out into its own function, and sprinkled references to Notes to better explain all of the moving parts. Many thanks to @simonpj for the suggestions.