[dev.typeparams] go/types: trigger verification while resolving instance

The refactoring of CL 335929 to merge the instance and Named types
resulted in type instances only being evaluated once. As a side effect,
we only verified constraints once per unique instantiation expression.

This can be confusing if type instantations are occurring far apart in
the code. Resolve this by lifting up the verification logic into
Instantiate and InstantiateLazy.

Change-Id: Icd5a482d097d983073955c62931441edfd92f5c2
Reviewed-on: https://go-review.googlesource.com/c/go/+/335978


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>