[dev.typeparams] cmd/compile/internal/types2: move struct checking into separate file

This only moves functionality from one file into another.
Except for import adjustments there are no changes to the
code.

Change-Id: I8dff41fe82693c96b09a152975c3fd1e3b439e8d
Reviewed-on: https://go-review.googlesource.com/c/go/+/321589


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