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

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.

Differential Revision: https://reviews.llvm.org/D122612