Generic vs non-generic SYCL

The SYCL programming API is split into two categories; generic SYCL and non-generic SYCL. Almost everything in the SYCL programming API is considered generic SYCL. However any usage of the enum class backend is considered non-generic SYCL and should only be used for SYCL backend specialized code paths, as the identifiers defined in backend are implementation-defined.

To ensure portability of SYCL applications any non-generic SYCL application code where the backend enum class is used, the expression must be guarded with a preprocessor #ifdef guard using the associated preprocessor macro to ensure that the SYCL application will compile even if the SYCL implementation does not support that SYCL backend being specialized for.