Improve atomics support

**Is your feature request related to a problem? Please describe**

Support `sycl::atomic_ref` int `sycl-mlir`.

**Describe the solution you would like**

- Have a `sycl.atomic_ref` type in the `sycl` dialect;
- Define related operations: `sycl.atomic.load`, `sycl.atomic.store`, `sycl.atomic.exchange`...
- Lower these operations to `llvm`, possibly through `spirv`.

**Additional context**

Tests under [this directory](https://github.com/intel/llvm-test-suite/tree/b6f9ac47f5891476c18dd4980eb957c1cf15a2d7/SYCL/AtomicRef) will come handy.