<codecvt>: wstring_convert leaks memory

Describe the bug
The standard says that wstring_convert is to delete the supplied facet, but we don't appear to be doing that. See https://eel.is/c++draft/depr.conversions.string#18

The current implementation puts the facet into a std::locale object which may or may not delete the facet on completion.

Command-line test case
STL version (git commit or Visual Studio version):

C:\Users\bion\Desktop>type test.cpp
#define _SILENCE_CXX17_CODECVT_HEADER_DEPRECATION_WARNING
#include <assert.h>
#include <locale>
#include <stdio.h>

int fancyAlive = 0;

class fancy_state {};
class fancy_cvt : public std::codecvt<wchar_t, char, fancy_state> {
public:
  explicit fancy_cvt(size_t refs = 0)
      : std::codecvt<wchar_t, char, fancy_state>(refs) {
    ++fancyAlive;
  }
  ~fancy_cvt() {
    --fancyAlive;
    puts("destroyed!");
  }

  fancy_cvt(const fancy_cvt &) = delete;
  fancy_cvt &operator=(const fancy_cvt &) = delete;
};

int main() {
  {
    fancy_cvt *p = new fancy_cvt(5);
    assert(fancyAlive == 1);
    std::wstring_convert<fancy_cvt> cvt(p);
  }
  assert(fancyAlive == 0);
  puts("main() ending");
}

C:\Users\bion\Desktop>cl /EHsc /W4 /WX /std:c++latest /nologo .\test.cpp
test.cpp

C:\Users\bion\Desktop>.\test.exe
Assertion failed: fancyAlive == 0, file .\test.cpp, line 30

C:\Users\bion\Desktop>

This is a dual of Microsoft-internal VSO-364730 / AB#364730.

vNext note: Resolving this issue will require breaking binary compatibility. We won't be able to accept pull requests for this issue until the vNext branch is available. See #169 for more information.

Start with the provided wstring_convert reproducer and the cited standard requirement in depr.conversions.string#18. Inspect the wstring_convert entry point and its locale ownership behavior, then verify that the supplied facet is destroyed when conversion ownership ends. The issue notes that compatibility constraints defer implementation until the vNext branch is available.