<format>: Copy basic_format_args in tuple formatters when needed

[frederick-vs-ja]

Fixes #4651.

Also

• avoids creating a temporary string when the minimum width is specified to 0 or not specified, completing the "Future work" in Measure display width in tuple formatter #4631;
• reorganizes _Fmt_it, _Fmt_wit, format_context, and wformat_context with consistent alias templates; and
• tests that functions taking non-constructible basic_format_context specializations can still be well-formed, despite that they can't be actually called.

Notes: LWG-4061 is now tentatively ready (although this PR doesn't speculatively implements it yet). Thus it should be now clear that basic_format_context objects can only be created by implementations, not user codes.

[cpplearner]

This doesn't really copy the arguments, right? It copies basic_format_args, which holds a pointer to the array of format_arg objects.

[StephanTLavavej]

Thanks! I pushed a conflict-free merge with main and some minor nitpicks.

[StephanTLavavej]

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

[StephanTLavavej]

Thanks for fixing the runtime correctness of one of the most popular new features! 🪄 🐈 🐱