Fix precision tracking of constructor return values

Precision should be set for constructor return values if they are of a
built-in type. Structs should not be precision qualified.

BUG=angle:787

Change-Id: Ie5efd5be25a788ff6f01c5b989254572c00231eb
Reviewed-on: https://chromium-review.googlesource.com/229560
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Nicolas Capens <capn@chromium.org>
Tested-by: Olli Etuaho <oetuaho@nvidia.com>