[WIP] [DO NOT MERGE] add std::span support

[cperkinsintel]

This PR depends upon the C++17 uplift which is not yet merged ( #3447 )

This PR adds std::span support to SYCL, however it is not yet ready to merge. It still needs tests and there are decisions about files and file management.

The files libcxx_span.hpp, libcxx_config.hpp and libcxx_undefmacros.hpp are all copies from Clang libcxx, which is not presently built or used by DPCPP. The original files were named span, __config and __undef_macros. In the case for libcxx_config.hpp and libcxx_undefmacros.hpp I have only lightly modified them, by converting statements like #include <__config> to #include "libcxx_config.hpp".

But libcxx_span has had more changes, such as dropping the private __wrap_iter class that LIBCXX defines in its implementation of <iterator>. I have inserted comments with //CP wherever I effected a change. Otherwise, I've tried to leave the files untouched - so they are as close to their original LIBCXX counterparts as possible.

Questions:

• do we want these file names ( libcxx_span.hpp etc) or should they be named something else?
• is it ok to put these three files in include/CL/sycl ?
• our SYCL header files use #pragma once but these header files all use the #ifndef X / #define X ... #endif trick. Any issues there?
• the libcxx_span file opens with a long synopsis comment. I have left that intact.

Anyway, let me know if you have feedback.

[Pennycook]

SYCL 2020 makes std::span available in the sycl:: namespace (see https://www.khronos.org/registry/SYCL/specs/sycl-2020/html/sycl-2020.html#sec:progmodel.futurecppversion). Do you want to do that as part of this PR?

[cperkinsintel]

@Pennycook - Yes, it needs to be moved to the sycl namespace and it should check for being available before including this. Thanks for reminding me.

[romanovvlad]

It seems the guard should be renamed as well. Otherwise it can guard away original span from libc++ user might want to include.

[cperkinsintel]

I've incorporated the feedback. The C++ uplift macro has been merged, so there is a new final PR for the span support here: #3569

I am closing this draft.