[ORC] Add ExecutionSession::removeJITDylibs (plural), use it in endSession.

The ExecutionSession::removeJITDylibs operation will remove all JITDylibs in
the given list (i.e. first clear them, then remove them from the session).

ExecutionSession::endSession is updated to remove JITDylibs rather than just
clearing them. This prevents new code from being added to any JITDylib once
endSession has been called.