[dev.regabi] cmd/compile: add Name.Canonical and move Byval

There's a bunch of code that wants to map closure variables back to
their original name, so add a single Name.Canonical method that they
can all use.

Also, move the Byval flag from being stored on individual closure
variables to being stored on the canonical variable.

Passes toolstash -cmp.

Change-Id: Ia3ef81af5a15783d09f04b4e274ce33df94518e6
Reviewed-on: https://go-review.googlesource.com/c/go/+/281541


Trust: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>