go/types,cmd/compile/internal/types2: better diagnostic for type shadowing

go/types,cmd/compile/internal/types2: better diagnostic for type shadowing

This change causes the "x is not a type" diagnostic to describe
x's actual kind, helping to reveal when shadowing is at work.

(The kind description could improve other errors too.)

Fixes #76877

Change-Id: Ia3484998bb384ff570c20b6792cf8461c60aa38c
Reviewed-on: https://go-review.googlesource.com/c/go/+/731180
Reviewed-by: Robert Griesemer <gri@google.com>
Auto-Submit: Alan Donovan <adonovan@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Robert Griesemer <gri@google.com>