[lldb] force full gui redraw on Ctrl+L

As is common with curses apps, this allows to redraw everything
in case something corrupts the screen. Apparently key modifiers
are difficult with curses (curses FAQ it "doesn't do that"),
thankfully Ctrl+key are simply control characters, so it's
(ascii & 037) => 12.

Differential Revision: https://reviews.llvm.org/D84972