[SYCL] Changes to vec/swizzle implementation

[aelovikov-intel]

• Remove expression trees functionality from swizzles following the clarifications made by the SYCL 2020 specification
• Make several previously templated functions non-templated (as they should be according to the specification) as well as aligning other availability constraints with the spec.
• Make named single element (.x()/.r()/etc.) return a swizzle vs previously returned reference to a scalar
• Unify implementation of certain features between sycl::vec and swizzles (named swizzles and multiple binary/unary/op-assign operators)
• Simplified implementation of the vec::(const ArgTys... &args) ctor

Deviations from the specifications added as part of this PR:

• Added non-standard copy-assignment operator for swizzle type. Otherwise, depending on the implementation it might either be implicitly deleted or implicitly defined (potentially with counterintuitive/unintended behavior)
• vec::(const ArgTys... &args) ctor - add an additional NumElements > 1 requirement to resolve the following ambiguity otherwise present with the current is_convertible implementation (a pre-existing deviation from the spec, see below):

  sycl::vec<sycl::half, 1> v{1};  // float/double are ok
  v = 2;     // ambiguous
  v.x() = 3; // ambiguous

Other known remaining differences from the specification inherited from the previous implementation:

• vec::(const ArgTys... &args) ctor requirements have been weaker (std::is_convertible_v<T, DataT>) than required by the specification (must be an exact match of the element_type). Also, unlike the specification, it allows args to be swizzles (in addition to scalars and vecs).
• vec(const vector_t &) ctor is still templated to avoid ambiguity with vec(const DataT &) for one-element vectors
• std::byte support, in particular how some hidden friend operators aren't available and non-standard shift operators are implemented instead, e.g. operator<<(const Self &, int)

Newly added test is mostly copied from @steffenlarsen 's #13026.