[serialization] Don't keep a ModuleFile around when missing dependencies.

All we need is the skeleton Module to let us know not to try again. This
was previously leading to a use-after-free, caught by Guard Malloc.
Thanks, Argyrios!

Swift SVN r6267

lib/Serialization/SerializedModuleLoader.cpp

@@ -211,6 +211,8 @@ Module *SerializedModuleLoader::loadModule(SourceLoc importLoc,
                            diag::serialization_missing_dependencies,
                            missingNames);
       }
+
+      module->File = nullptr;
     }
   }