_Locinfo: Use _wsetlocale to query and restore locales

[lb90]

_Locinfo changes the locale temporarily and then reverts to the previous locale on destruction. The sequence of setlocale calls are as follows:

1. oldlocname = setlocale(LC_ALL, nullptr) to query the locale string
2. setlocale(LC_ALL, newlocname) to set the temporary locale
3. setlocale(LC_ALL, oldlocname) to restore the previous locale

However there's a catch here: the fully-qualified locale names returned by setlocale are not always ASCII strings (more on that below). This creates challenges because the oldlocname is encoded depending on the "outer" locale, while the setlocale call at point 3) expects an encoding which follows the "inner" locale, and the two may not match.

To solve this issue, use the wide variant of setlocale: _wsetlocale. This way all strings are UTF-16 and there's no issue with varying narrow string encodings.

Addendum:

Actually, the C RunTime library does its best to use ASCII strings! It queries the english name of the locale using GetLocaleInfoEx. MSDN says that the returned string is always ASCII [1], but that's not always the case [2].

Fixes #5780

References:

1. https://learn.microsoft.com/en-us/windows/win32/intl/locale-senglish-constants
2. https://developercommunity.visualstudio.com/t/GetLocaleInfoEx-w-LOCALE_SENGLISHLANGUA/10981789

I made a similar change in libc++: llvm/llvm-project#160479

[StephanTLavavej]

Thank you! 😻 (And apologies for taking so long to review this, <flat_meow> took a while.) I am cautious because locales/iostreams burned me repeatedly at the beginning of my career, but this looks like a solid fix and if it goes wrong it'll be in a surprising way which is all I can ask for. 😹