cmd/compile: pass an explicit position to namedAuto

cmd/compile: pass an explicit position to namedAuto

To enable this, inline the call to nod and simplify.
Eliminates a reference to lineno from the backend.

Passes toolstash-check -all.

Updates #15756

Change-Id: I9c4bd77d10d727aa8f5e6c6bb16b0e05de165631
Reviewed-on: https://go-review.googlesource.com/38441
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>