Having only one way of setting or clearing arrayness of types clarifies
code dealing with arrays.

TEST=angle_unittests
BUG=angleproject:941

Change-Id: I98cb7c44fd66440c9de8b4c6c4a02827e9300db7
Reviewed-on: https://chromium-review.googlesource.com/264361Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Olli Etuaho <oetuaho@nvidia.com>