`<locale>`: Calling `use_facet<collate<char>>(loc)` changes output of `strftime()` in a different thread

[StephanTLavavej]

Originally reported as DevCom-246257 / internal VSO-275595 by constructing regex{"meow"}, but regex calls use_facet so the repro can be reduced:

Important

This repro requires you to temporarily modify your system's locale settings.

C:\Temp>type meow.cpp

// Settings > Time & language > Date & time > Change the date and time format > Long time
// Change "9:40:07 AM / 2:40:07 PM" to "09:40:07 / 14:40:07"

#include <clocale>
#include <cstdio>
#include <cstring>
#include <ctime>
#include <locale>
#include <thread>
using namespace std;

void run_on_other_thread() {
    locale loc{};
    (void) use_facet<collate<char>>(loc);
}

int main() {
    tm t{};
    t.tm_hour = 14;
    t.tm_min  = 30;
    t.tm_sec  = 45;

    setlocale(LC_ALL, "");

    char out1[100];
    strftime(out1, sizeof(out1), "%X", &t);
    puts(out1);

    {
        jthread thread(&run_on_other_thread);
    }

    char out2[100];
    strftime(out2, sizeof(out2), "%X", &t);
    puts(out2);

    if (strcmp(out1, out2) == 0) {
        puts("PASS");
    } else {
        puts("FAIL");
    }
}

C:\Temp>cl /EHsc /nologo /W4 /std:c++latest /MTd /Od /Zi /Fdmeow.pdb meow.cpp
meow.cpp

C:\Temp>meow
14:30:45
2:30:45 PM
FAIL

[cpplearner]

#5785 seems related.