[SYCL] Add namespace alias for cl::sycl namespace

[bader]

The alias is enabled by default and make using SYCL API a little bit
easier.

Unfortunately it's not compatible with the following valid C++ code:

namespace sycl {
  using namespace cl::sycl;
}

In order to disable namespace alias user should define
__SYCL_DISABLE_NAMESPACE_ALIAS__ macro.

Signed-off-by: Alexey Bader [email protected]

[keryell]

This seems dangerous.
If you want to use some alternate namespaces, why not including alternate headers instead of breaking existing code since you decide anyway to change the code to use for example ::sycl instead of cl::sycl::
Look at for example https://github.com/triSYCL/triSYCL/blob/master/tests/sycl_namespace/demo_parallel_matrix_add.cpp