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: I09f2f99b3108e0d2fb1919a0631e4317b6a28be3
Reviewed-on: https://chromium-review.googlesource.com/263709Reviewed-by: Olli Etuaho <oetuaho@nvidia.com>
Tested-by: Olli Etuaho <oetuaho@nvidia.com>