Emulating arc hyperbolic functions is required on HLSL, where they do
not exist natively. For this, BuiltInFunctionEmulator is split into GLSL
and HLSL subclasses. The GLSL subclass handles the pre-existing built-in
emulation implemented for working around OSX bugs, and the HLSL subclass
handles emulating asinh, acosh and atanh on HLSL.

BUG=angle:855

Change-Id: I0dfeffb862ac27ba7f9ecf5492ec31d9d952b273
Reviewed-on: https://chromium-review.googlesource.com/236861Reviewed-by: Nicolas Capens <capn@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Olli Etuaho <oetuaho@nvidia.com>