Introduce ASTConsumer::HandleTopLevelDeclInObjCContainer which accepts

top-level declarations that occurred inside an ObjC container.

This is useful to keep track of such decls otherwise when e.g. a function
is declared inside an objc interface, it is not passed to HandleTopLevelDecl
and it is not inside the DeclContext of the interface that is returned.

llvm-svn: 142232