[dev.typeparams] cmd/compile/internal/types: review of gccgosizes.go

Except for the package name, this file is unchanged from the go/types version.

f=gccgosizes.go; diff $f ../../../../go/types/$f
8c8
< package types2
---
> package types

Change-Id: I23a8432f3e6f21eec8220f89a24df26e91ad41ab
Reviewed-on: https://go-review.googlesource.com/c/go/+/265697


Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>