Use std::function instead of llvm::function_ref for a callback field.

According to the documentation, llvm::function_ref is better suited for function parameters taking closures.
But in this case we need to store a closure in the callback field and then invoke it multiple times. Therefore it is more appropriate to use std::function for this purpose.

include/swift/Serialization/ModuleFile.h

@@ -79,7 +79,7 @@ class ModuleFile : public LazyMemberLoader {
   StringRef IdentifierData;
 
   /// A callback to be invoked every time a type was deserialized.
-  llvm::function_ref<void(Type)> DeserializedTypeCallback;
+  std::function<void(Type)> DeserializedTypeCallback;
 
 public:
   /// Represents another module that has been imported as a dependency.