Rather than just on or off, I've changed it to allow "off", "fast",
or "full". "fast" means that we'll do symbol lookup, but we won't
try to find inline frames and we won't run line number programs
(those are the things that are taking considerable time in some
cases).
rdar://122302117
Symbolication can take some time, depending on the binaries involved.
In certain contexts it's better for the backtrace to finish quickly,
and then symbolication could be done offline.
rdar://122302117
We shouldn't try to erase the message when in non-color mode; mostly
that's intended for redirected output scenarios anyway, and in that case
we don't really want garbage in the output.
Also, improve the failure messages when the backtracer itself goes
wrong or can't be executed.
Finally, change the behaviour slightly so that *if* we're explicitly
enabled, *and* the backtracer path wasn't explicitly specified, *and*
we can't find the backtracer, we print a warning on start-up. We
don't do that in any other case because we don't want spurious warnings
everywhere.
rdar://118055527
When we crash, emit a message straight away that says we're working
on a backtrace. If starting the backtracer fails, report that also.
Finally, add a duration to the messages output by the backtracer, so
that we can see how long it took.
rdar://118055527
