This change adds constant folding support for unary common built-ins:
abs, sign, floor, trunc, round, roundEven, ceil and fract.

BUG=angleproject:913
TEST= dEQP tests
dEQP-GLES3.functional.shaders.constant_expressions.builtin_functions.common*
(80 out of 210 tests started passing with this change)

Change-Id: I46312fec43084601d4fca8195ddaaa5292f1c02a
Reviewed-on: https://chromium-review.googlesource.com/265967Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>