go/types: simplify Named.under
go/types: simplify Named.under

This is a straighforward port of CL 341857 to go/types.

Change-Id: I3407676232b595662c1470627771a13263703061
Reviewed-on: https://go-review.googlesource.com/c/go/+/342479
Trust: Robert Findley <9179eee875d1ee628c37fb47d7e749e855ffd21f@google.com>
Run-TryBot: Robert Findley <9179eee875d1ee628c37fb47d7e749e855ffd21f@google.com>
Reviewed-by: Robert Griesemer <a6752097faebf1f02d80107d77a9be009089afe3@golang.org>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

nawawi/go,golang/go,cloudflare/go,nawawi/go,golang/go,nawawi/go,nawawi/go,cloudflare/go,nawawi/go,nawawi/go,golang/go,CAFxX/go,golang/go,cloudflare/go,cloudflare/go,CAFxX/go,CAFxX/go,CAFxX/go,CAFxX/go,cloudflare/go,nawawi/go,golang/go,golang/go,CAFxX/go,golang/go,nawawi/go,CAFxX/go,golang/go,cloudflare/go,cloudflare/go,CAFxX/go,cloudflare/go