[lldb] Add support for specifying a log handler

This patch adds a new flag to `log enable`, allowing the user to specify
a custom log handler. In addition to the default (stream) handler, this
allows using the circular log handler (which logs to a fixed size,
in-memory circular buffer) as well as the system log handler (which logs
to the operating system log).

Differential revision: https://reviews.llvm.org/D128323