[lldb] Fix statusline terminal resizing
Simplify and fix the logic to clear the old statusline when the terminal
window dimensions have changed. I accidentally broke the terminal
resizing behavior when addressing code review feedback.
I'd really like to figure out a way to test this. PExpect isn't a good
fit for this, because I really need to check the result, rather than the
control characters, as the latter doesn't tell me whether any part of
the old statusline is still visible.
