go/types, types2: disable field accesses through type parameters

go/types, types2: disable field accesses through type parameters

This is a feature that is not understood well enough and may have
subtle repercussions impacting future changes. Disable for Go 1.18.

The actual change is trivial: disable a branch through a flag.
The remaining changes are adjustments to tests.

Fixes #51576.

Change-Id: Ib77b038b846711a808315a8889b3904e72367bce
Reviewed-on: https://go-review.googlesource.com/c/go/+/391135
Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>