[dev.regabi] cmd/compile: unexport Node.RawCopy

[dev.regabi] cmd/compile: unexport Node.RawCopy

RawCopy breaks the invariant that ir.Orig depends on for
allowing nodes to omit keeping their own orig fields.
Avoid surprises by unexporting it.

The only use in package gc was removed in the previous CL.
This one is a straight global search and replace RawCopy -> rawCopy.

Change-Id: Ia99c0f4665bf7ed4f878cc44456d5fbdf33bab8d
Reviewed-on: https://go-review.googlesource.com/c/go/+/274293
Trust: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>