[dev.inline] cmd/internal/obj: use src.Pos in obj.Prog

This will let us use the src.Pos struct to thread inlining
information through to obj.

Change-Id: I96a16d3531167396988df66ae70f0b729049cc82
Reviewed-on: https://go-review.googlesource.com/34195
Run-TryBot: David Lazar <lazard@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>
Reviewed-by: Austin Clements <austin@google.com>

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

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