[OpenCL] Supports optional generic address space semantics in C++ for OpenCL 2021

Adds support for a feature macro `__opencl_c_generic_adress_space`
in C++ for OpenCL 2021 enabling a respective optional core feature
from OpenCL 3.0. Testing is only performed in SemaOpenCL because
generic address space functionality is yet to be implemented in
C++ for OpenCL 2021.

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

Differential Revision: https://reviews.llvm.org/D108461