Implement LWG-3631 basic_format_arg(T&&) should use remove_cvref_t<T> throughout

[frederick-vs-ja]

Also make formerly exposition only constructors private. As a result, several class templates are forward declared.

Add test coverage for construction of handle and inaccessibility of exposition only constructor(s). Note that construction of handle from a const rvalue is still possible since a const T rvalue can be bound to const T&.

Per @cpplearner's discovery on Discord, I decided to ensure that TQ (_Qual_for_handle<_Ty>_Tq in this PR) never drops const-qualifier from T (_Ty).

Fixes #3460.

[barcharcraz]

This changes behavior pretty radically in the case of stuff that has conversions to one of the types supported "natively" by basic_format_arg, I actually like the change but just want to note that potentially breaking change. It's probably good though, since now such types can get formatted by their own formatter.

Requesting only the change to inline _Qual_for_handle

[barcharcraz]

This changes behavior pretty radically in the case of stuff that has conversions to one of the types supported "natively" by basic_format_arg, I actually like the change but just want to note that potentially breaking change. It's probably good though, since now such types can get formatted by their own formatter.

Requesting only the change to inline _Qual_for_handle

[StephanTLavavej]

Thanks! FYI @barcharcraz I pushed trivial changes after you approved.

[StephanTLavavej]

I'm mirroring this to the MSVC-internal repo - please notify me if any further changes are pushed.

[StephanTLavavej]

Thanks for improving the usability of <format>! 😻 📝 🎉