I would like to add a

    func (t *Type) Elem() *Type

method to package gc, but that would collide with the existing

    func (t *Type) Elem() ssa.Type

method needed to make *gc.Type implement ssa.Type.  Because the latter
is much less widely used right now than the former will be, this CL
renames it to ElemType.

Longer term, hopefully gc and ssa will share a common Type interface,
and ElemType can go away.

Change-Id: I270008515dc4c01ef531cf715637a924659c4735
Reviewed-on: https://go-review.googlesource.com/20546
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>