[MLIR] Fix AttributeInterface declaration.

[ulysseB]

Summary

Substitues Type by Attribute in the declaration of AttributeInterface. It
looks like the code was written by copy-pasting the definition of TypeInterface,
but the substitution of Type by Attribute was missing at some places.

[rriddle]

Ooops, thanks for fixing!

mlir/include/mlir/IR/Attributes.h

 /// This class represents the base of an attribute interface. See the definition
 /// of `detail::Interface` for requirements on the `Traits` type.
 template <typename ConcreteType, typename Traits>
 class AttributeInterface
     : public detail::Interface<ConcreteType, Attribute, Traits, Attribute,
                                AttributeTrait::TraitBase> {
 public:
   using Base = AttributeInterface<ConcreteType, Traits>;
-  using InterfaceBase = detail::Interface<ConcreteType, Type, Traits, Type,
-                                          AttributeTrait::TraitBase>;
+  using InterfaceBase = detail::Interface<ConcreteType, Attribute, Traits,
+                                          Attribute, AttributeTrait::TraitBase>;
   using InterfaceBase::InterfaceBase;
 
 private:
   /// Returns the impl interface instance for the given type.
   static typename InterfaceBase::Concept *getInterfaceFor(Attribute attr) {
     return attr.getAbstractAttribute().getInterface<ConcreteType>();
   }