[dev.regabi] cmd/compile: change noder.declNames to returns ir.Names

declNames always returns a slice of *ir.Names, so return that directly
rather than as []ir.Node.

While here, also change iimport to directly create ir.ODCL/ir.OAS
statements, rather than calling variter. Allows eliminating a use of
ir.TypeNode.

Passes buildall w/ toolstash -cmp.

Change-Id: Icb75e993c4957b6050c797ba64ee71cfb7a19644
Reviewed-on: https://go-review.googlesource.com/c/go/+/279315


Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Russ Cox <rsc@golang.org>