[lldb] Show progress events in the command line driver

This patch adds support for showing progress events when using lldb on
the command line. It spawns a separate thread that listens for progress
events and prints them to the debugger's output stream.

It's nothing fancy (yet), for now it just prints the progress message.
If we know the total number of items being processed, we prefix the
message with something like [1/100], similar to ninja's output.

This patch doesn't use any fancy terminal manipulation: it uses a simple
carriage return (\r) to bring the cursor to the front of the line and
vt100 escape codes to clear the (rest) of the line.

Differential revision: https://reviews.llvm.org/D120972