go/types, types2: generalize cleanup phase after type checking

Use a cleanup method and simple registration mechanism
for types that need some final processing before the end
of type checking.

Use cleanup mechanism instead of expandDefTypes mechanism
for *Named types. There is no change in functionality here.

Use cleanup mechanism also for TypeParam and Interface types
to ensure that their check fields are nilled out at the end.

Introduce a simple constructor method for Interface types
to ensure that the cleanup method is always registered.

In go/types, add tracing code to Checker.checkFiles to match
types2.

Minor comment adjustments.

Fixes #51316.
Fixes #51326.

Change-Id: I7b2bd79cc419717982f3c918645af623c0e80d5e
Reviewed-on: https://go-review.googlesource.com/c/go/+/387417


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