[OpenCL] Supports optional writing to 3d images in C++ for OpenCL 2021

Adds support for a feature macro __opencl_c_3d_image_writes in
C++ for OpenCL 2021 enabling a respective optional core feature
from OpenCL 3.0.

This change aims to achieve compatibility between C++ for OpenCL
2021 and OpenCL 3.0.

Differential Revision: https://reviews.llvm.org/D109328