go/ast: fix broken build with typeparams build constraint

My rebase of https://golang.org/cl/300649 before submitting broke the
build (and tests) when using the typeparams build constraint. In a
subsequent CL I add test coverage back to cmd/dist.

This time, I've tested by running:
 - go test -tags=typeparams go/...
 - go test -tags=typeparams cmd/gofmt

All tests pass except for the new TestResolution/typeparams.go2, which I
will fix in a follow-up CL.

For #44933

Change-Id: I439d387841604cf43a90e2ce41dbe6bbbdb0306d
Reviewed-on: https://go-review.googlesource.com/c/go/+/310070


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>