We can trust that untyped composite literals are part of a slice literal
and not emit a vet warning for those.

Fixes #9171

Change-Id: Ia7c081e543b850f8be1fd1f9e711520061e70bed
Reviewed-on: https://go-review.googlesource.com/22000
Run-TryBot: Rob Pike <r@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rob Pike <r@golang.org>