[SYCL-Spec] According to [SYCL-2020 spec](...

According to SYCL-2020 spec, the types used for kernel names be a C++ type and be forward declarable. Since Eigen is the expression tree-based kernel, the expression type will be used as the name of the kernel. When an enum is used in the kernel's name, the integration header in SYCL must forward enum. To do this forward declaration, either the unscoped enum must have an underlying type specified or the scoped enum (e.g enum class) should be used. however, using scoped enum requires the enumerator to be qualified by enum type(e.g EnumCLASS::VALUE) which can be a more intrusive change. This PR inherits the enum from int to be less intrusive and compliant with the SYCL2020 spec for kernel name.