go/types, types2: better error reporting for Checker.implements

This CL copies (and adjusts as needed) the logic for error reporting
from operand.assignableTo to Checker.implements in the case of a missing
method failure and assignment to an interface pointer.

Preparation for using Checker.implements in operand.assignableTo
rather than implementing the same logic twice.

This also leads to better errors from Checker.implements as it's
using the same logic we already use elsewhere.

For #50646.

Change-Id: I199a1e02cf328b222ae52c10131db871539863bf
Reviewed-on: https://go-review.googlesource.com/c/go/+/381434


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