[Modules] Fix an obvious lack of deterministic ordering when processing

rewritten decls for Objective-C modules.

Found by inspection and completely obvious, so no test case. Many of the
remaining determinism fixes won't have precise test cases at this point,
but these are the kinds of things we wouldn't ask for a specific test of
during code review but ask authors to fix. The functionality isn't
changing, and should (he he!) already be tested.

llvm-svn: 233333