go/types, types2: disallow new methods on (aliases to) cgo-generated types
This makes methods on aliases of cgo-generated types a new compiler error.
That is ok because cgo-behavior is not covered by the G1 compatibility
guarantee.
Background: In 2023 we fixed a gopls issue related to this by actually
enabling methods on cgo-generated types in the first place (#59944).
See the discussion in #60725 and this CL for why we believe it is ok
to make this an error now.
Based on a variation of CL 503596 (by Xie Cui).
Fixes #60725.
For #59944.
Change-Id: I7e9e6e1a76447167483a282b268f5183214027c9
Reviewed-on: https://go-review.googlesource.com/c/go/+/629715 Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Robert Griesemer <gri@google.com> Reviewed-by: Robert Findley <rfindley@google.com> Reviewed-by: Robert Griesemer <gri@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>