[SYCL] Align usm_allocator ctor and operators with SYCL2020

[KseniyaTikhomirova]

Signed-off-by: Tikhomirova, Kseniya kseniya.tikhomirova@intel.com

[KseniyaTikhomirova]

I found that I am not able to build simple piece of code with SYCL RT by gcc 9.3.0 but able to build using clang.

The error I receive using gcc is because CLassA constructor and operators are declared as noexcept default while constructors and operators for some class members of classA are not declared as no except explicitly. And according to https://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#1778 gcc generates error.
I also found that this behavior with deletion should be reverted in c++20 and this proposal was accepted https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/p1286r2.html

The classes I am pointing to are:
usm_allocator with copy constructor which explicitly declared as noexcept
https://github.com/intel/llvm/blob/sycl/sycl/include/sycl/usm/usm_allocator.hpp#L42
which has device as field and device's copy constructor is not explicitly specified as noexcept:
https://github.com/intel/llvm/blob/sycl/sycl/include/sycl/device.hpp#L83

I clarified with front end engineer if our usage could be ill-formed and it shouldn't:
device class has single field which is shared_ptr and it is copy constructor is explicitly noexcept. So device could be also noexcept but implicitly.

Nevertheless I suggest to align constructors and operators declaration with spec and nested objects. Because extra noexcept adds extra limitation to constructor and makes not possible to build some valid code with gcc 9.3.0 (as example).