RFC: Update Intel386, x86-64 and IA MCU psABIs for passing/returning empty struct

[Matthijs van Duin via cfe-commits]

To avoid depending again on precise wording of definitions in C++
standard it may be worth being explicit about the requirement to be
trivially copyable *and* destructible, since although the former
implies the latter in the C++ standard this is not obvious from the
terminology (although you also need the latter in all practical cases
including the one being discussed).

The explanation I gave earlier ("it means an object can be copied
simply by copying its bytes and destroyed simply by discarding its
storage") can probably be polished into a language-agnostic
definition.

I think it is helpful in general when ABI standards can be extended to
other languages with as much ease and unambiguity as possible, for the
sake of interoperability.

For similar reasons I would support "aggregate occupying zero bytes
(excluding any padding)" with a footnote on the strange situation of
C/C++ arrays, which cannot be directly passed by value even if the
syntax may suggest they can (hence the rule being discussed is not
applicable to them).

Matthijs van Duin