Support for new namespace

[romanovvlad]

This patch aims to remove cl:: namespace from sycl/sycl.hpp header.
Currently sycl/sycl.hpp exposes SYCL API in cl::sycl namespace. It needs to be changed to just sycl:: so the user code which includes this header and accesses something using cl::sycl should produce an error.

The patch moves all SYCL APIs to a new __sycl_internal namespace and imports API from it to cl::sycl if CL/sycl.hpp in included or just to sycl in case of sycl/sycl.hpp:

namespace __sycl_internal {
  class buffer;
  ...
}
namespace sycl {
  using namespace __sycl_internal;
}

FE and llvm passes are updated accordingly to check for new namespace where it's needed.
In order to keep backward compatibility the patch adds aliases for each exported function from cl::sycl namespace to __sycl_internal namespace.

The patch is going to be committed in several pieces:

1. Introduce a macro which will be defined to old namespace(cl::sycl)
2. Change FE to recognize SYCL entities from both cl::sycl and __sycl_internal namespaces and start using the macro during integration header/footer instead of cl::sycl
3. Similar to 2), but for llvm passes
4. Change all usages of cl::sycl with usages of the macro
5. Replace cl::sycl with __sycl_ns (which at this stage will be the same)
6. Work with exceptions
7. Change macro to use new namespaces and update tests

[denis-kabanov]

/summary:run

[denis-kabanov]

/summary:run

[denis-kabanov]

/summary:run

[denis-kabanov]

/summary:run