[dev.typeparams] cmd/compile: provide scaffolding to get types2 types during noding

Initial setup of types2.Info structure to provide access to types
computed by generic typechecker.

Use -G flag to control compiler phases with new typechecker:

-G    (or -G=1)  parsing and typechecking ony
-G -G (or -G=2)  parsing, typechecking, and noding
-G=3             continue after noding (currently will run old
                 typechecker again, leading to duplicate errors

Change-Id: I87dd54f7c3773228f288f7a134ac809d9481ca95
Reviewed-on: https://go-review.googlesource.com/c/go/+/274444


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