[OpenCL] Supports optional same image reads and writes in C++ for OpenCL 2021

Adds support for a feature macro `__opencl_c_read_write_images` 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/D109307