Support `char8_t` in C++17 mode

[MahmoudGSaleh]

The STL currently understands and handles char8_t in C++17 and lower language modes when compiling with /Zc:char8_t.
To complete the support for p0482r6 - char8_t: A type for UTF-8 characters and strings), the following is still needed:

• Add test coverage for P0482 under /std:c++17 /Zc:char8_t
• Support u8string and u8string_view when __cpp_char8_t is defined (not only in C++20 mode)
• Determine which (if any) of the changes from WG21-P1423 we should down-port as well.

Related to DevCom-1030065.

[StephanTLavavej]

We plan to work on this after completing the C++20 Defect Reports.

[ingo-loehken]

I'd just like to ask if your plans to bring in something like /Zc:lib_char8_t as feature toggle compiler vendor supported equivalent to hacking /D__cpp_lib_char8_t into /std:c++17 Standard Compiler Options is in schedule after C++ 20 DRs have been finished ?

If so, do you plan to target vs2022 and backport to vs2019 ?

Another question off-topic: I did not find any proposals for std::u8stringstream or similar in the C++ Standard. This hinders Interoperability and with char8_t and signedness of char in common differs. Are there any plans from Microsoft STL Team to fill that gap ?

[fsb4000]

@ingo-loehken u8string, u8string_view and std::filesystem additions will be available with /Zc:char8_t in VS 2022 17.4 Preview 1 or 2.
I don't know if it will be backported to vs2019.
About std::u8stringstream, as far as I know there is no such proposal from Microsoft employees and they usually implement what C++ standard says.
https://github.com/microsoft/STL#non-goals