<mdspan>: More EBO

[JMazurkiewicz]

• Add new base classes required for EBO implementation:
  • _Maybe_fully_static_extents - EBO for layout_left::mapping,layout_right::mapping and layout_stride::mapping,
  • _Mdspan_mapping_base - EBO for mdspan (_Map member only),
  • _Mdspan_accessor_base - EBO for mdspan (_Acc member only),
• Add /* [[no_unique_address]] */ comments when EBO cannot be applied.
  • Ideally we would get rid of those base classes and just use this attribute (see Use [[no_unique_address]] after Clang supports it on Windows #1364).
• Drive-by: address comment from <mdspan>: Make extents truly empty when rank_dynamic() == 0 #3825 (static_cast<size_t> in _Calculate_rank_dynamic).
• Add tests.

[StephanTLavavej]

Thanks! Noticed a couple of issues in test comments that can be fixed in a followup PR, but they weren't worth resetting testing. 😻