Ignore transitive ObjC imports when cross-importing

This behavior change reduces the chance of unexpected and unwanted cross-imports being performed.

Fixes rdar://problem/60554019.

include/swift/AST/Module.h

@@ -448,7 +448,7 @@ public:
   /// Retrieve the top-level module. If this module is already top-level, this
   /// returns itself. If this is a submodule such as \c Foo.Bar.Baz, this
   /// returns the module \c Foo.
-  ModuleDecl *getTopLevelModule();
+  ModuleDecl *getTopLevelModule(bool overlay = false);
 
   bool isResilient() const {
     return getResilienceStrategy() != ResilienceStrategy::Default;