go/types: temporarily pin the Checker to Interface during checking

While type checking expressions involving interface types, it is
possible that their type set is used before delayed actions are
processed. As a result, computeInterfaceTypeSet is called with a nil
checker, and errors in the interface type definition result in panics
(see #48234).

To avoid the panics, store a *Checker on Interface for use in between
checking of the interface type expression and processing of delayed
actions.

Fixes #48234

Change-Id: I5509bc1c01b55edac52446b9e075fbe8fcc01874
Reviewed-on: https://go-review.googlesource.com/c/go/+/348371


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