[lldb] Deprecate SBHostOS threading functionality

For some context, Raphael tried to this before: https://reviews.llvm.org/D104231

These methods are not tested at all, and in some cases, are not even fully
implemented (e.g. SBHostOS::ThreadCreated). I'm not convinced it's
possible to use these correctly from Python, and I'm not aware of any
users of these methods. It's difficult to remove these methods
wholesale, but we can start with deprecating them.

A possible follow-up to this change (which may require an RFC to get
more buy in from the community) is to gut these functions entirely. That
is, remove the implementations and replace them either with nothing or
have them dump out a message to stderr saying not to use these.

Differential Revision: https://reviews.llvm.org/D153900