Addition of the entry points for GL_EXT_clip_control extension.

Bug: angleproject:5471
Change-Id: If17ba0120bdb226cc5728bfea83e9085260eba19
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2615862Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Mohan Maiya <m.maiya@samsung.com>