Add `[[trivial_abi]]` attribute

[philnik777]

Clang implements [[clang::trivial_abi]] (docs) to allow passing classes that are non-trivial for the purposes of calls as-if they were trivial. For example, this allows passing std::unique_ptrs in registers instead of on the stack, which results in a significant performance and code size win (https://godbolt.org/z/aTsG34abP).

It would be nice to standardize the attribute to allow it to be used across compilers.

I think it should be enough to modify the definition of non-trivial for the purposes of calls to something like:

A type is considered non-trivial for the purposes of calls if:

• it isn't marked [[itanium::trivial_abi]], or
• the type directly declares a virtual base of virtual member functions, or
• all of its copy and move constructors are deleted, or
• the type has a base class that is non-trivial for the purposes of calls, or
• the type has a non-static data member whose type is non-trivial for the purposes of calls, which includes:
  • classes that are non-trivial for the purposes of calls,
  • arrays of classes that are non-trivial for the purposes of calls
• and
  • it has a non-trivial copy constructor, move constructor, or destructor, or
  • all of its copy and move constructors are deleted.

This mostly is the list of things when [[clang::trivial_abi]] is ignored. Suggestions to simplify the definition are welcome. Should there be a note that classes marked [[trivial_abi]] have to be trivially relocatable (move + destroy are equivalent to a memcpy)?

CC @ldionne @jwakely