sycl: use complex extension

[bd4]

The fact that std::complex works at all in device code is specific to Intel's SYCL implementaiton, and not part of standard SYCL. Standardizing it is in process, with a header only implementation available here: https://github.com/argonne-lcf/SyclCPLX

To aid in the standardization process and make sure this will work for our needs, we should explore using this instead of std::complex for the SYCL backend. Eventually it should be possible to use this with Intel without the extra header, i.e. this will be included in the open source llvm fork and in the official oneAPI releases eventually.