This adds new built-ins found in ESSL 3.10 section 8.4 Floating-Point
Pack and Unpack Functions.

This includes constant folding support and support for both GLSL and
HLSL output. In HLSL all of these functions need to be emulated.

BUG=angleproject:1730
TEST=angle_unittests
TEST=dEQP-GLES31.functional.shaders.*pack*norm4x8*

Change-Id: Ibed60286a366cd35c4faafd405e79af562a02a06
Reviewed-on: https://chromium-review.googlesource.com/434170
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>