[dev.regabi] cmd/compile: simplify declaration importing

Rather than creating Names w/ ONONAME earlier and later adding in the
details, this CL changes the import logic to create and add details at
the same time.

Passes buildall w/ toolstash -cmp.

Change-Id: Ifaabade3cef8cd80ddd6644bff79393b934255d9
Reviewed-on: https://go-review.googlesource.com/c/go/+/279313


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