[OpenCL] Define OpenCL 3.0 optional core features in C++ for OpenCL 2021

Modifies OpenCL 3.0 optional core feature macro definitions so that
they are set analogously 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/D108704