[SYCL] Change KernelType definition to use std::decay_t

[slawekptak]

Change the way KernelType is defined, so the const and reference declarations are removed. Also, use std::decay_t and/or detail::nth_type_t where applicable.

[aelovikov-intel]

While on it, I'd suggest using

llvm/sycl/include/sycl/detail/type_traits.hpp

Lines 405 to 419 in 282e8cc

	#if __has_builtin(__type_pack_element)
	template <int N, typename... Ts>
	using nth_type_t = __type_pack_element<N, Ts...>;
	#else
	template <int N, typename T, typename... Ts> struct nth_type {
	using type = typename nth_type<N - 1, Ts...>::type;
	};
	template <typename T, typename... Ts> struct nth_type<0, T, Ts...> {
	using type = T;
	};
	template <int N, typename... Ts>
	using nth_type_t = typename nth_type<N, Ts...>::type;
	#endif

and change to std::decay_t<detail::nth_type_t<RestT...>>.