<format>: Simplify type-erased argument storage

[CaseyCarter]

Rename the overload set that emulates the exposition-only constructors of basic_format_arg from _Get_format_arg_storage_type to _Phony_basic_format_arg_constructor for clarity. Encapsulate that overload set - and the traits _Storage_type and _Storage_size that observe it - in a new class template _Format_arg_traits. Remove the bogus monostate case and split the floating-point types out from the function template overload to more closely reflect the basic_format_arg constructors.

Shorten _Format_arg_store_packed_index to _Format_arg_index and make its converting constructor unconditionally noexcept.

In _Format_arg_store, store indices in an array using the untyped storage only for erased arguments. In _Store, reuse _Format_arg_traits::_Storage_type instead of trying to duplicate the overload resolution process.

Add test coverage for visit_format_arg(/**/, basic_format_arg</**/>()) to ensure that the monostate is properly visitable.

[barcharcraz]

I thought you could store monostates and observe them through visit_format_arg? If that's no longer the case then we should also move monostate back to

[CaseyCarter]

I thought you could store monostates and observe them through visit_format_arg? If that's no longer the case then we should also move monostate back to

You can visit_format_arg(my_visitor, basic_format_arg<some_context>()), which notably doesn't involve basic_format_args or _Format_arg_store. Users create a _Format_arg_store by passing arguments to make_format_args; there's no argument they can pass that results in storing a monostate. (Note that monostate itself is not a valid argument type for make_format_args because there's no enabled formatter specialization for monostate.)

EDIT: I note that we are missing test coverage of visit_format_arg(my_visitor, basic_format_arg<some_context>()), which makes me nervous - I'll add some.

[StephanTLavavej]

Looks good - I will validate and push changes for ultra-trivial nitpicks.

[StephanTLavavej]

Thanks for simplifying <format> - a net deletion of 56 lines of product code! 😻 ✏️