[SYCL-2020 Support] Enabling Intel DPCPP Compiler support to Eigen
Additional information
This PR enables intel opensource DPCPP compiler for Eigen SYCL backend. The C++17 features have been enabled as it is the requirement of SYCL-2020 features. Using this backend, it is possible to compile and run Eigen SYCL backend on a set of accelerators including (but not limited to) IntelCPU, IntelGPU, NvidiaGPU, etc