[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>

src/cmd/compile/internal/gc/main.go

@@ -229,10 +229,10 @@ func Main(archInit func(*Arch)) {
 	loadsys()
 
 	timings.Start("fe", "parse")
-	lines := parseFiles(flag.Args(), base.Flag.G != 0)
+	lines := parseFiles(flag.Args())
 	timings.Stop()
 	timings.AddEvent(int64(lines), "lines")
-	if base.Flag.G != 0 {
+	if base.Flag.G != 0 && base.Flag.G < 3 {
 		// can only parse generic code for now
 		base.ExitIfErrors()
 		return