[dev.typeparams] cmd/compile: add "check" field to noder.gcimports

The unified IR importer needs access to the *types2.Checker instance
to lazily construct objects and types. Eventually, maybe the
types2.Importer API can be extended to add the Checker as another
parameter (or more likely something like an ImportConfig struct), but
right now we can handle this ourselves as long as we forgo the
types2.(*Config).Check convenience wrapper.

Updates #46449.

Change-Id: I89c41d5d47c224a58841247cd236cd9f701a23a1
Reviewed-on: https://go-review.googlesource.com/c/go/+/327053


Trust: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>