[lldb] Replace Host::SystemLog with Debugger::Report{Error,Warning}

As it exists today, Host::SystemLog is used exclusively for error
reporting. With the introduction of diagnostic events, we have a better
way of reporting those. Instead of printing directly to stderr, these
messages now get printed to the debugger's error stream (when using the
default event handler). Alternatively, if someone is listening for these
events, they can decide how to display them, for example in the context
of an IDE such as Xcode.

This change also means we no longer write these messages to the system
log on Darwin. As far as I know, nobody is relying on this, but I think
this is something we could add to the diagnostic event mechanism.

Differential revision: https://reviews.llvm.org/D128480