[dev.inline] cmd/compile/internal/syntax: process //line pragmas in scanner

Reviewed in and cherry-picked from https://go-review.googlesource.com/#/c/33764/.

Minor adjustment in noder.go to make merge compile again.

Change-Id: Ib5029b52b59944f207b0f2438c8a5aa576eb25b8
Reviewed-on: https://go-review.googlesource.com/34233


Run-TryBot: Robert Griesemer <gri@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>