Add a traverser that checks precision qualifiers of folded constants and
hoists them to separate precision qualified variables if needed.

Fixes sdk/tests/conformance/glsl/bugs/constant-precision-qualifier.html

TEST=WebGL conformance tests, angle_unittests
BUG=angleproject:817

Change-Id: I1639595e0e49470736be93274f0af07ee732e1fe
Reviewed-on: https://chromium-review.googlesource.com/275095Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Olli Etuaho <oetuaho@nvidia.com>