[SYCL] SYCL 2020 standalone device selectors ( gpu_selector_v and friends)

[cperkinsintel]

implementation of standalone selectors (e.g. gpu_selector_v, etc)

new tests for llvm-test-suite are here: https://github.com/intel/llvm-test-suite/pull/1141

[cperkinsintel]

/verify with https://github.com/intel/llvm-test-suite/pull/1141

[cperkinsintel]

For some reason, when resolving the merge conflicts, there were only conflict in the linux abi .dump file, not the Windows one. Odd. I merged, rebuilt on both platforms, and reran the ABI symbols generation on both. The merge conflict is resolved, but on Windows my .dump has picked up the symbols from the recently merged https://github.com/intel/llvm/pull/6474 for some reason. Maybe reordering? Maybe the symbols from 6474, while correct, weren't generated by the python script?

[cperkinsintel]

/verify with https://github.com/intel/llvm-test-suite/pull/1141

[cperkinsintel]

ping to reviewers.

[cperkinsintel]

ping to reviewers and mergers. I have had to update this twice already because of linux symbol merge conflicts. My worry is that if it takes too long it'll get conflicted again.