[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>