Look into LLDB support

[luser]

LLDB implements the GDB protocol, but we should see if there are any quirks we'd need to support:
https://github.com/llvm-mirror/lldb/blob/master/docs/lldb-gdb-remote.txt

[tromey]

According to rr-debugger/rr#2078, lldb doesn't advertise the multiprocess feature. If that's still the case then we'd have to rethink this area a bit.