Addition of shader autogen code for EXT_clip_cull_distance extension.

Bug: angleproject:5458
Change-Id: I299528957c0e747fd84987c420588c314f54aae0
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2590989
Commit-Queue: Mohan Maiya <m.maiya@samsung.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>