[libcxx] [test] Fix back-to-back use of get_temp_file_name() on Windows

[mstorsjo]

Summary

On non-Windows platforms, get_temp_file_name() uses mkstemp(),
which picks a unique name and creates a file atomically. The
Windows implementation uses _mktemp_s(), which doesn't create the
file. The documentation of _mktemp_s() also says that by design,
the function uses the same pattern within a process, as long as that
file doesn't exist.

Thus previously, two consecutive calls to get_temp_file_name()
on Windows returned the same file name.

Try to create the suggested temp file with _O_EXCL (marking the file name as already used for future calls to _mktemp_s) and retry if we weren't able to exclusively create the file.

This fixes the test failures on Windows observed in D122257.

[mstorsjo]

Ping @ldionne. This would avoid needing exceptions/xfails for windows in D122257.

[mstorsjo]

Made the temp file creation atomic by creating the suggested temp file with _open(_O_CREAT | _O_EXCL).

libcxx/test/support/platform_support.h

 #endif
 
 inline
 std::string get_temp_file_name()
 {
 #if defined(_WIN32)
     char Name[] = "libcxx.XXXXXX";
     if (_mktemp_s(Name, sizeof(Name)) != 0) abort();
+    FILE *File = fopen(Name, "wb");
+    if (File)
+        fclose(File);
     return Name;
 #else
     std::string Name;
     int FD = -1;
     do {
         Name = "libcxx.XXXXXX";
         FD = mkstemp(&Name[0]);
         if (FD == -1 && errno == EINVAL) {