go/types, types2: don't crash in selectors referring to the type being declared

In Checker.typInternal, the SelectorExpr case was the only case that
didn't either set or pass along the incoming def *Named type.

Handle this by passing it along to Checker.selector and report a
cycle if one is detected.

Fixes #51509.

Change-Id: I6c2d46835f225aeb4cb25fe0ae55f6180cef038b
Reviewed-on: https://go-review.googlesource.com/c/go/+/390314


Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>