`<locale>`: std::locale alters the global C-locale

[dalle]

Describe the bug

Constructing a std::locale with a different locale encoding than previously passed to setlocale also alters the global C-locale. This is especially obvious when having set an alternative decimal separator.

Command-line test case

PS C:\dev\test_locale> type test_locale.cpp
#include <locale>
#include <stdio.h>
#include <locale.h>

constexpr auto value = 1.1;

int main()
{
   printf("setlocale:     %s\n", setlocale(LC_ALL, ".utf8"));
   puts("");

   printf("decimal_point: %s\n", localeconv()->decimal_point);
   printf("printf:        %g\n", value);
   puts("");

   {
      const auto loc = std::locale("");
   }

   printf("decimal_point: %s\n", localeconv()->decimal_point);
   printf("printf:        %g\n", value);
}
PS C:\dev\test_locale> cl /EHsc /W4 /WX /std:c++latest .\test_locale.cpp
Microsoft (R) C/C++ Optimizing Compiler Version 19.44.35217 for x86
Copyright (C) Microsoft Corporation.  All rights reserved.

/std:c++latest is provided as a preview of language features from the latest C++
working draft, and we're eager to hear about bugs and suggestions for improvements.
However, note that these features are provided as-is without support, and subject
to changes or removal as the working draft evolves. See
https://go.microsoft.com/fwlink/?linkid=2045807 for details.

test_locale.cpp
Microsoft (R) Incremental Linker Version 14.44.35217.0
Copyright (C) Microsoft Corporation.  All rights reserved.

/out:test_locale.exe
test_locale.obj
PS C:\dev\test_locale> .\test_locale.exe
setlocale:     English_United States.utf8

decimal_point: '
printf:        1'1

decimal_point: .
printf:        1.1

Expected behavior

I would expect std::locale to not alter the global C-locale, regardless of which encoding is used.

STL version

Microsoft (R) C/C++ Optimizing Compiler Version 19.44.35217 for x86

Additional context

When running this program I have set the Time & language > Language & region > Regional format to "English (United States)", and I have altered the Regional format > Number format > Decimal symbol to a single quote '.

[dalle]

Closely related to #5780