[SYCL] Do not use sycl::cl_* types in vec class (#8149)

Refactored `vec`'s underlying storage type selection so it doesn't
reference `sycl::cl_*` types which do not exists in SYCL 2020 spec
anymore.

New approach takes less lines of code and it also provides better error
messages for wrong data type or number of elements passed by user.

The change is mostly non-functional, except for the new error messages.

Author: AlexeySachkov

sycl/include/sycl/detail/generic_type_traits.hpp

@@ -41,6 +41,8 @@ template <typename T> using is_halfn = is_contained<T, gtl::vector_half_list>;
 
 template <typename T> using is_genfloath = is_contained<T, gtl::half_list>;
 
+template <typename T> using is_half = is_contained<T, gtl::scalar_half_list>;
+
 template <typename T>
 using is_svgenfloath = is_contained<T, gtl::scalar_vector_half_list>;