[OPENCL] opencl-c.h: add initial CL 3.0 conditionals for atomic operations.

This adds the optional wrappers around things, however this isn't sufficient yet for CL 3.0 without generic address space, I've got one more additional patch to add all those APIs, but this is an easier to review precursor.

Reviewed By: Anastasia

Differential Revision: https://reviews.llvm.org/D106111