[dev.typeparams] cmd/compile: enable type-checking of generic code

This change makes a first connection between the compiler and types2.
When the -G flag is provided, the compiler accepts code using type
parameters; with this change generic code is also type-checked (but
then compilation ends).

Change-Id: I0fa6f6213267a458a6b33afe8ff26869fd838a63
Reviewed-on: https://go-review.googlesource.com/c/go/+/264303


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