Move all front end functions that are only called from the OpenCL
entry points to a separate section at the top of the front end
object class bodies, which improves readability.

Bug: angleproject:6001
Change-Id: Id360adbf0c439e4068c379aa4ea1da25e99f7b53
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2928419Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Cody Northrop <cnorthrop@google.com>