go/types, types2: use factored parse/typecheck functions, generate hilbert_test.go

go/types, types2: use factored parse/typecheck functions, generate hilbert_test.go

Change-Id: I4a325736d18a98bbcd02bfa3d32b1d1dd2048dc2
Reviewed-on: https://go-review.googlesource.com/c/go/+/461609
Auto-Submit: Robert Griesemer <gri@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>
Run-TryBot: Robert Griesemer <gri@google.com>