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: I2b7a61320819dcd095827faa1fd16e835f4688b4
Reviewed-on: https://chromium-review.googlesource.com/265819Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>