An untyped constant can be defined in any input file, we shouldn't
segregate them by file.

Updates #28772

Change-Id: I0347f15236833bb511eb49f86c449ee9241b0a25
Reviewed-on: https://go-review.googlesource.com/c/151600
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Andrew Bonventre <andybons@golang.org>