This change adds constant folding support for trigonometry built-in
functions. Constant folding for these functions also fixes constant
expression issues where constant initializer is a built-in trignometry
function whose arguments are all constant expressions.

BUG=angleproject:913
TEST= dEQP tests
dEQP-GLES3.functional.shaders.constant_expressions.builtin_functions.angle_and_trigonometry*
(112 out of 120 tests pass with this change)

Change-Id: I2ed4360532469fe0d70048d5f2300a8db6f9fcda
Reviewed-on: https://chromium-review.googlesource.com/263679Reviewed-by: Olli Etuaho <oetuaho@nvidia.com>
Tested-by: Olli Etuaho <oetuaho@nvidia.com>