[SYCL] Provide SYCL 2020 function objects

[maximdimakov]

Provided sycl function objects accordingly to sycl 2020 specification
The test for function objects in test_suite - intel/llvm-test-suite#310
Signed-off-by: mdimakov maxim.dimakov@intel.com

[Pennycook]

I don't think we should be importing extensions into the sycl:: namespace via aliases. There are several (subtle) ways in which a SYCL 2020 feature may differ from its ONEAPI equivalent, and we need to be very careful.

Also, I think the specific algorithm you're adding here was already added as part of #3786.

[maximdimakov]

sycl::reduce_over_group was merged yesterday. The only sycl::plus is remained

[sergey-semenov]

There are several other function objects that are in SYCL 2020 but are currently provided only in ONEAPI namespace just like plus: multiplies, bit_and, etc. I think it makes sense to handle them all in a single patch since the changes are quite minor, but I don't mind if those are going to be added separately.

Minor comment: I think it makes more sense to provide sycl::plus in another file in the sycl directory rather than sycl/ONEAPI/.

[maximdimakov]

@sergey-semenov I added functional objects accordingly to 4.17.2 point of spec. Is stl.hpp suitable for these function objects?

[sergey-semenov]

Is stl.hpp suitable for these function objects?

I think creating sycl/functional.hpp for these would be more appropriate.

[maximdimakov]

The test in test_suite - intel/llvm-test-suite#310

[sergey-semenov]

LGTM overall