We encode the state of looking that the parent scope is a Class into a
type so that we can avoid extra lookups. Then use that in refactoring
where diagnostics are generated for an explicit `Self` in an `extend
impl` declaration.
We add tests that we don't double-diagnose the Self type when it's
already an error, and make the behaviour of `extend require` match that
of `extend impl as`.
This avoids some fragile/complex parse-node lookups (such as
`context.parse_tree_and_subtrees().ExtractAs<Parse::ImplTypeAs>`) by
using the parse node at the point where we are handling it instead of
much later.
This is part of #6420 which is being split up into a chain of smaller
PRs.
