Spec: Improve Definition of Type Layout

[chorman0773]

This improves the definition of Type Layout to be more formal. The following changes are made:

• The guarantees of repr(Rust) structs and unions are defined in a separate section, as they apply to all structs and unions, even when they have another representation.
• The layout of repr(Rust) enums is documented, with some permissions granted conservatively to implementations (like not including uninhabited variants in the size/alignment of the enum), pending approval either way.
  • I'm not too attached to including these, but omitting them could be taken as a prohibition against them in the future.
• Discriminant Elision (now guaranteed by Finish stabilization of result_ffi_guarantees rust#130628) is documented.
  • This is intended the supersede update the docs for layout guarantees of option-like enums unsafe-code-guidelines#538.
• The repr(C) layouts are consolidated, though the section on structs is left presently. I'm not sure whether to get rid of it or replace it with a large note admonition, because it isn't needed as normative text, but the definition of repr(C) as applied to structs can be confusing on its own.

I think there's some more work to be done on the chapter, but I think this is a reasonable set of stuff to do in one PR, specifically targetted at the repr attribute.

[chorman0773]

Link check errors are intentional in bea8b9d - I link to sections from #1618 and omitted the footnot references so it can be reverted when #1618 is merged.