Restore the signal handler for Windows as it is not presistent there.

On windows, signal handler is reset to default once a signal is received. 
This causes doing ctrl-c twice on console (or pressing suspend button twice
in eclipse ide which uses SIGINT to stop the debuggee) to crash lldb-mi on 
windows. Although there is very tiny window (after signal handler is called
and before we restore the handler) where default handler will be in place.
But this is hardly a problem in practice as IDEs generally disable their suspend
button once it has been presses.

llvm-svn: 227964