cmd/internal/obj: remove ATYPE

cmd/internal/obj: remove ATYPE

In cmd/compile, we can directly construct obj.Auto to represent local
variables and attach them to the function's obj.LSym.

In preparation for being able to emit more precise DWARF info based on
other compiler available information (e.g., lexical scoping).

Change-Id: I9c4225ec59306bec42552838493022e0e9d70228
Reviewed-on: https://go-review.googlesource.com/36420
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Heschi Kreinick <heschi@google.com>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>