go/types: delay expansion of underlying in typeDecl

go/types: delay expansion of underlying in typeDecl

Even after type-checking the RHS of a type declaration, we may not yet
be able to expand, if the RHS is itself an instance (see #49043).

We can instead rely on the mechanisms we have in place for delayed
expansion.

Fixes #49043

Change-Id: Ibffa4c1b1163c824b5c7e151aaac35f3e8c84ec7
Reviewed-on: https://go-review.googlesource.com/c/go/+/356533
Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
Reviewed-by: Robert Griesemer <gri@golang.org>