cmd/vet: use type info to detect the atomic funcs

cmd/vet: use type info to detect the atomic funcs

Simply checking if a name is "atomic" isn't enough, as that might be a
var or another imported package. Now that vet requires type information,
we can do better. And add a simple regression test.

Change-Id: Ibd2004428374e3628cd3cd0ffb5f37cedaf448ea
Reviewed-on: https://go-review.googlesource.com/91795
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
Reviewed-by: Robert Griesemer <gri@golang.org>