[dev.inline] cmd/compile: introduce cmd/internal/src.Pos type for line numbers

This is a step toward chosing a different position representation.
By introducing an explicit type, it will be easier to make the
transition step-wise while ensuring everything keeps running.

This has been reviewed via https://go-review.googlesource.com/#/c/34025/.

Change-Id: Ibceddcd62d8f346321ac3250e3940e9c436ed684
Reviewed-on: https://go-review.googlesource.com/34132
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Lazar <lazard@golang.org>

src/cmd/compile/internal/gc/gsubr.go

@@ -45,7 +45,7 @@ func Prog(as obj.As) *obj.Prog {
 	}
 
 	p.As = as
-	p.Lineno = lineno
+	p.Lineno = int32(lineno) // TODO(gri) fix this
 	return p
 }