go/types, types2: add the Interface.MarkImplicit method

Add a new interface method, MarkImplicit, to allow marking interfaces as
implicit from outside the type-checker. This is necessary so that we can
capture the implicit bit in export data, and use it from importers.

For #48424
For #49040

Change-Id: I999aba2a298f92432326d7ccbd87fe133a2e1a72
Reviewed-on: https://go-review.googlesource.com/c/go/+/357796


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>