AST: Introduce ModuleDecl::ImportFilter::ForLinking

ModuleDecl::forAllVisibleModules() now has a includeLinkOnlyModules
parameter. This is intended to be used when computing the set of
libraries to autolink.

lib/Serialization/ModuleFile.cpp

@@ -1598,6 +1598,13 @@ void ModuleFile::getImportedModules(
         continue;
 
       break;
+
+    case ModuleDecl::ImportFilter::ForLinking:
+      // FIXME
+      if (!dep.isExported())
+        continue;
+
+      break;
     }
 
     assert(dep.isLoaded());