[lldb] Report debugger diagnostics as events

Report warnings and errors through events instead of printing directly
the to the debugger's error stream. By using events, IDEs such as Xcode
can report these issues in the UI instead of having them show up in the
debugger console.

The new diagnostic events are handled by the default event loop. If a
diagnostic is reported while nobody is listening for the new event
types, it is printed directly to the debugger's error stream.

Differential revision: https://reviews.llvm.org/D121511