go/types, types2: simplify missingMethodReason

Added a funcString helper so we don't need to rewrite strings
with strings.Replace.

Use compiler format for error message about wrong method type;
this removes another unnecessary variation.

Simplify conditions for pointer-to-interface related error:
if one of the involved types is an interface pointer, it can't
have any methods.

Rewrite logic so we don't need all the else-if branches.

Adjusted a test case for types2 accordingly. The go/types version
of this test case has a different error because the implementation
of Checker.typeAssertion is different in the two type checkers
(the types2 version gives errors closer to the 1.17 compiler).

Change-Id: I19e604d7063c3e31e8290bd78384a9f38bb0d740
Reviewed-on: https://go-review.googlesource.com/c/go/+/382694


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