[lldb] Make some functions useful to REPLs public

`StartEventHandlerThread` and `StopEventHandlerThread` are available to the SwiftREPL even though they are protected because SwiftREPL is a friend class of Debugger. I'm developing my own REPL and having access to these functions, including `FlushProcessOutput`, is desirable.

Differential Revision: https://reviews.llvm.org/D149717