[OpenCL] Supports optional program scope global variables in C++ for OpenCL 2021

Adds support for macro `__opencl_c_program_scope_global_variables`
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/D109305