This patch adds "/dev/tty" as a special output file name for lit tests.
If a lit test has a RUN line that includes a redirection to "/dev/tty",
the redirection goes to the special device file corresponding to the
console. It is /dev/tty on UNIX-like systems and "CON" on Windows.
This patch is needed to implement a test like PR25717 (caused by
the size limit of the Windows system call WriteConsole() prior to
Windows 8) where the test only breaks when outputing to the
console and won't fail if using a pipe.