Add a hard limit to how many frames lldb will unwind in a single
thread before UnwindLLDB::AddOneMoreFrame calls it quits.  We have
a couple of reports of unending backtraces in the field and we
haven't been able to collect any information about what kind of
backtrace is causing this.  We've found on Mac OS X that it's tricky
to get more than around 200k stack frames before a process exceeds
its stack space so we're starting with a hard limit of 300,000 frames.
<rdar://problem/13383069>