[lldb] Emit diagnostic events in the diagnostic dump

[lldb] Emit diagnostic events in the diagnostic dump

Connect the diagnostic events with the diagnostic infrastructure.

 - Emit existing diagnostic events (warnings and errors) to the
   diagnostic log.
 - Introduce a new diagnostic event (info) that's used exclusively for
   diagnostic logging and does not get broadcast.

Differential revision: https://reviews.llvm.org/D136648