swift234

[ClangImporter] Make lookupVisibleDecls deterministic.

Clang's LookupVisibleDecls is implemented by iterating over the map of lookups,
which clearly isn't deterministic. This *shouldn't* be a problem, but it
seems our importer has non-determinism based on the order it sees decls.
For now, at least, let's make this deterministic up front so that we don't get
weird results from :print_module.

See <rdar://problem/15785883> and <rdar://problem/15799697>.

Swift SVN r12309