go/types: ensure that Named.check is nilled out once it is expanded

To support lazy expansion of defined types, *Named holds on to a
*Checker field, which can pin the *Checker in memory. This can have
meaningful memory implications for applications that keep type
information around.

Ensure that the Checker field is nilled out for any Named types that are
instantiated during the type checking pass, by deferring a clean up to
'later' boundaries.

In testing this almost exactly offset the ~6% memory footprint increase
I observed with 1.17.

Fixes #45580

Change-Id: I8aa5bb777573a924afe36e79fa65f8729336bceb
Reviewed-on: https://go-review.googlesource.com/c/go/+/318849


Trust: Robert Findley <rfindley@google.com>
Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>