go/types, types2: always use missingMethodReason in checker.Implements

Remove special case where we don't have a *Checker and always use
Checker.missingMethodReason in Checker.implements.

Look for zero methods rather than empty interface to exit early
from Checker.missingMethod, and remove the extra test in
Checker.implements.

With this change we get consistent and more detailed error messages
from all places where we do a form of the "implements" test.

To make this possible, allow for the receiver to be nil in
- Checker.sprintf
- Checker.missingMethodReason
- Checker.interfacePtrError
- Checker.funcString

Allowing Checker.sprintf with nil Checker permits further simplifying
in a couple of places.

Change-Id: I0ea7178c9efbcd4a25ded2a66e2b058db52dc4d4
Reviewed-on: https://go-review.googlesource.com/c/go/+/383054


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