Reduce verbosity of address spaces for builtin functions in OpenCL

While writing https://github.com/KhronosGroup/OpenCL-Docs/pull/358 I added a lot of overloads of atomic builtin functions for different address spaces.

The OpenCL C spec currently uses `gentype` language to reduce duplication of definitions of builtin functions that only vary by type. The same approach could be added for address spaces, something like `genaddrspace`. This was previously suggest on an OpenCL tooling call.

This could be done initially for the atomic functions that inspired this. In principal it may help with other builtins as well.

It would need to be clear the `gentype` and `genaddrspace` combine to create a cross-product of instantiations.

Also atomics operate on `{global, local}` (`private` is still in discussion on https://github.com/KhronosGroup/OpenCL-Docs/issues/331),but non-atomic operations that can read from memory are generally defined for `{global, local, private, constant}` but ones that can write to memory generally only for `{global, local, constant}`.

Start by reviewing the OpenCL C specification's atomic builtin definitions and the related discussion in pull request 358 and issue 331. Define how gentype and the proposed genaddrspace combine, including the permitted address-space sets for atomic and non-atomic builtins. Done means the atomic definitions are less repetitive without changing their intended address-space coverage.

Written by the indexing model from the issue text.