cmd/compile: enable -G=3 by default

This CL changes cmd/compile's -G flag's default from 0 to 3, which
enables use of the new types2 type checker and support for type
parameters. The old type checker is still available with
-gcflags=all=-G=0.

The CL also updates the regress test harness to account for the change
in default behavior (e.g., to expect known types2 changes/failures).
However, the -G=0 mode is still being tested for now.

Copy of CL 340914 by danscales@, minus the cmd/internal/objabi.AbsFile
change (handled instead by CL 343731) and rebased to master branch.

Updates #43651.

Change-Id: I1f62d6c0a3ff245e15c5c0e8f3d922129fdd4f29
Reviewed-on: https://go-review.googlesource.com/c/go/+/343732


Trust: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>