[OpenCL] Fix parsing of opencl-c.h in CL 3.0

Ensure that the cl_khr_3d_image_writes pragma is enabled by making
cl_khr_3d_image_writes an optional core feature in CL 3.0 in addition
to being an available extension in 1.0 onwards and a core feature in
CL 2.0.

https://reviews.llvm.org/D99425



Signed-off-by: Kevin Petit <kevin.petit@arm.com>