[SYCL] Simplify check whether a header file is available for inclusion

[dyniols]

__has_include is part of the C++ standard since C++17: https://en.cppreference.com/w/cpp/preprocessor/include

We require C++17 if sycl.hpp is included, so we can use __has_include without extra checks for its presence.

[steffenlarsen]

While you're at it, could we apply the same to all other uses of defined(__has_include)? I see it in sycl/source/detail/os_util.cpp and sycl/include/syclcompat/kernel.hpp.

[dyniols]

While you're at it, could we apply the same to all other uses of defined(__has_include)? I see it in sycl/source/detail/os_util.cpp and sycl/include/syclcompat/kernel.hpp.

Sure