[swift2objc] Support parsing multiple modules

[liamappelbe]

This should be pretty easy to implement. We'll end up with a symbolgraph json for each, passing a List<Json> to parseAst, and we basically just merge all their symbols and relations into one big ParsedSymbolgraph, before passing it to parseDeclarations as normal.

Also parse Foundation by default, in addition to the user's requested modules. Without these changes, most modules will fail to parse because they depend on stuff from Foundation.

[liamappelbe]

When we're parsing Foundation by default, we need to make sure that we don't generate bindings for the Foundation APIs, unless they're needed as a transitive dep. The transitive deps check is related to filtering, so I'll wait for that to land before implementing this.