Add "/dev/tty" as a special 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.

llvm-svn: 258898