[OpenCL][NFC] opencl-c.h: reorder atomic operations

This just reorders the atomics, it doesn't change anything except their layout in the header.

This is a prep patch for adding some conditionals around these for CL3.0 but that patch is much easier to review if all the atomic operations are grouped together like this.

Reviewed By: Anastasia

Differential Revision: https://reviews.llvm.org/D105601