Introduce SYCL 2020 mode

Currently, we have support for SYCL 1.2.1 (also known as SYCL 2017).
This patch introduces the start of support for SYCL 2020 mode, which is
the latest SYCL standard available at (https://www.khronos.org/registry/SYCL/specs/sycl-2020/html/sycl-2020.html).
This sets the default SYCL to be 2020 in the driver, and introduces the
notion of a "default" version (set to 2020) when cc1 is in SYCL mode
but there was no explicit -sycl-std= specified on the command line.