go/types: guard against checking instantiation when generics is disabled

When type checking t[_], where t is a type name, it was possible to leak
an error message related to generics. Fix this by guarding on
typeparams.Enabled.

In order to test this fix, we need to be able to run the new go/types
test only if type parameters are disabled. Introduce the .go1 test data
suffix (similar to .go2) to control this behavior.

Originally found via fuzzing, though the test case was manually
simplified.

Updates #46404

Change-Id: Ib1e2c27cf974c2a5ca5b9d6d01b84a30ba4d583b
Reviewed-on: https://go-review.googlesource.com/c/go/+/329793


Trust: Robert Findley <rfindley@google.com>
Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>