[SYCL] Make half default constructor constexpr

[AlexeySachkov]

Unfortunately, it seem to be impossible to have a trivial default constexpr constructor until we enable C++23 mode. Therefore, this patch made a few other adjustments to account for non-trivial default constructor of half class.

[AlexeySachkov]

Note for reviewers: I honestly don't know why would we need a trivial constructor here, but if necessary, we can return that check back and add a special case for half like we used to have, see 909a824

[AlexeySachkov]

Note for reviewers: this is a non-trivial constructor! std::is_trivially_default_constructible_v<sycl::half> is false.

Reason for that is because trivially constructible classes can't have non-static members with default initializers. From cppreference:

Trivial default constructor
The default constructor for class T is trivial (i.e. performs no action) if all of the following is true:

• The constructor is not user-provided (i.e., is implicitly-defined or defaulted on its first declaration)
• T has no non-static members with default initializers. (since C++11)

We have non-static member Buf with default initializer below and we can't remove it or otherwise we can't make the constructor constexpr.

Without default initializer for Buf, the following error will be emitted: defaulted definition of default constructor is not constexpr. The reason for that is because constexpr constructors must have all fields initialized: cppreference:

constexpr constructor
A constexpr constructor whose function body is not =delete; must satisfy the following additional requirements:

• for the constructor of a class or struct, every base class sub-object and every non-variant non-static data member must be initialized.

Apparently, trivial default constexpr constructors are only available in C++23: P1331R2.

Not making the default constructor trivial brings several issues (of different impact):

• sycl::bit_cast<sycl::half> doesn't work anymore, because it requires To type to be trivially default constructible. I don't see any reason for such strict requirement and therefore I'm relaxing it in this PR

• putting sycl::half into union requires a bit more work from the user: from cppreference:

  If a union contains a non-static data member with a non-trivial default constructor, the default constructor of the union is deleted by default unless a variant member of the union has a default member initializer.

  That effectively means that user has to either provide default constructor for union containing sycl::half or provide default member initializer for sycl::half member to make it compile. You can find both approached used in this PR to adjust both implementation and tests.

• device_global currently requires trivially default constructible types, i.e. it means we can't have device_global<sycl::half>:

  llvm/sycl/include/sycl/ext/oneapi/device_global/device_global.hpp

  Line 128 in 3d8a136

  static_assert(std::is_trivially_default_constructible<T>::value,

[AlexeySachkov]

Superseded by #8503