_LocInfo: Use _wsetlocale to query and restore locales

_LocInfo changes the locale temporarily and then reverts to the
previous locale on destruction. The sequence of setlocale calls
look 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 depend on 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

Author: lb90

stl/inc/xlocinfo

@@ -382,7 +382,7 @@ private:
     _Yarn<char> _Months; // month names
     _Yarn<wchar_t> _W_Days; // wide weekday names
     _Yarn<wchar_t> _W_Months; // wide month names
-    _Yarn<char> _Oldlocname; // old locale name to revert to on destruction
+    _Yarn<wchar_t> _Oldlocname; // old locale name to revert to on destruction
     _Yarn<char> _Newlocname; // new locale name for this object
 };
 _STD_END

stl/src/locale.cpp

@@ -137,9 +137,7 @@ void __CLRCALL_PURE_OR_CDECL locale::_Locimp::_Locimp_Addfac(
 
 void __CLRCALL_PURE_OR_CDECL _Locinfo::_Locinfo_ctor(
     _Locinfo* pLocinfo, int cat, const char* locname) { // capture a named locale
-    const char* oldlocname = setlocale(LC_ALL, nullptr);
-
-    pLocinfo->_Oldlocname = oldlocname == nullptr ? "" : oldlocname;
+    pLocinfo->_Oldlocname = _wsetlocale(LC_ALL, nullptr);
     _Locinfo_Addcats(pLocinfo, cat, locname);
 }
 

stl/src/locale0.cpp

@@ -222,9 +222,8 @@ void __CLRCALL_PURE_OR_CDECL locale::_Locimp::_Locimp_dtor(_Locimp* _This) { //
 
 void __CLRCALL_PURE_OR_CDECL _Locinfo::_Locinfo_ctor(
     _Locinfo* pLocinfo, const char* locname) { // switch to a named locale
-    const char* oldlocname = setlocale(LC_ALL, nullptr);
+    pLocinfo->_Oldlocname = _wsetlocale(LC_ALL, nullptr);
 
-    pLocinfo->_Oldlocname = oldlocname == nullptr ? "" : oldlocname;
     if (locname != nullptr) {
         locname = setlocale(LC_ALL, locname);
     }
@@ -233,9 +232,7 @@ void __CLRCALL_PURE_OR_CDECL _Locinfo::_Locinfo_ctor(
 }
 
 void __CLRCALL_PURE_OR_CDECL _Locinfo::_Locinfo_dtor(_Locinfo* pLocinfo) { // destroy a _Locinfo object, revert locale
-    if (!pLocinfo->_Oldlocname._Empty()) {
-        setlocale(LC_ALL, pLocinfo->_Oldlocname._C_str());
-    }
+    _wsetlocale(LC_ALL, pLocinfo->_Oldlocname._C_str());
 }
 _STD_END