[libc++] Android temp dir is /data/local/tmp, enable Windows test

[libc++] Android temp dir is /data/local/tmp, enable Windows test

On Android, std::filesystem::temp_directory_path() should fall back to
/data/local/tmp when no environment variable is set. There is no /tmp
directory. Most apps can't access /data/local/tmp, but they do have a
"cache dir" (Context#getCacheDir()) that is usable for temporary files.
However, there is no obvious and reliable way for libc++ to query this
directory in contexts where it is available. The global fallback
/data/local/tmp is available for "adb shell", making it useful for test
suites.

On Windows, temp_directory_path falls back to the Windows directory
(e.g. "C:\Windows"), so call GetWindowsDirectoryW to do the test.

Reviewed By: ldionne, #libc, enh

Differential Revision: https://reviews.llvm.org/D137131