mingw-w64: std::wcout after _setmode(_fileno(stdout), _O_WTEXT) doesn't work

[longnguyen2004]

Bugzilla Link	47302
Version	10.0
OS	Windows NT
CC	@mclow

Extended Description

Take this code snippet:

#include <iostream>
#include <io.h>
#include <fcntl.h>

int wmain(int argc, const wchar_t* argv[])
{
    _setmode(_fileno(stdout), _O_WTEXT);
    std::wcout << L"Thử nghiệm\n";
    return 0;
}

On MS STL, this correctly outputs the string, but on libc++ it outputs nothing.

Additional notes:
Add -municode when compiling with clang, and add /utf-8 when compiling with MSVC

[longnguyen2004]

In contrast, the following equivalent C code works on both:
#include <stdio.h>
#include <io.h>
#include <fcntl.h>

int wmain(int argc, const wchar_t* argv[])
{
_setmode(_fileno(stdout), _O_WTEXT);
_putws(L"Thử nghiệm\n");
return 0;
}

[mstorsjo]

I've posted an initial patch that tries to fix this issue at https://reviews.llvm.org/D146398.