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. 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.

Bumps the haddock submodule.

1 files changed, 0 insertions, 0 deletions

diff --git a/utils/haddock b/utils/haddock
-Subproject 07f2ca98fd4249dc6ebad053bd6aef90c814efe
+Subproject e7586f005aa89a45b0fc4f3564f0f17ab9f79d3