LWG-3810: CTAD for basic_format_args

[JMazurkiewicz]

Closes #3414.

[StephanTLavavej]

No change requested, note to other reviewers: Deduction guides can't be exported for modules, they're just automatically available if the class is exported (which is the case here), so the lack of any _EXPORT_STD is normal.

[StephanTLavavej]

I've pushed a commit to address @CaseyCarter's suggestion (using braces for the temporary, and applying [[maybe_unused]] to args_store even though compilers don't warn yet that it's unused at runtime). What persuaded me is that now we don't have to reason about the behavior of a test-only variable template. (Fewer lines is nicer but not decisive.)

[StephanTLavavej]

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

[StephanTLavavej]

Thanks for implementing more Cat Template Argument Destruction, where cats bat fragile template arguments off the table to smash on the floor! 🐈 🐾 😼