Strengthen exception specifications for stream types

[frederick-vs-ja]

This PR partially addresses #3278, but also strengthens exception specifications for some other functions. References to working draft are also updated to WG21-N4928.

Move assignment operators and swap functions are generally marked with noexcept. But I think move assignment operators of basic_filebuf and basic_(i|o)fstream shouldn't be noexcept, because they call basic_filebuf::close which may in turn call virtual functions that are possibly overridden. In other words, exception may be thrown from these move assignment operators. basic_syncbuf's should neither be noexcept due to LWG-3498.

Many move constructors remain non-noexcept, because they need to copy-construct a basic_streambuf, and hence need to dynamically allocate storage for locale. Maybe we should fix this in vNext.

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Currently EDG wrongly calculates std::is_nothrow_move_assignable_v<std::osyncstream> and std::is_nothrow_move_assignable_v<std::wosyncstream> as false, so these two static_asserts are skipped for EDG.

[strega-nil-ms]

Apart from the noexcept on osfx, this looks good; I can't guarantee I didn't miss something, though, since it's quite a lot of code...

[StephanTLavavej]

@strega-nil-ms FYI, I pushed a conflict-free merge with main, 3 comment changes for stable names, one this-> removal, and one 🐿️ removal 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 strengthening the STL! 🦾 🏋️ 📈