[mlir] Add constBuilderCall to DictionaryAttr

[jpienaar]

Let's at least have a test exercising this (with non-empty dictionary).

mlir/include/mlir/IR/OpBase.td

 }
 
 //===----------------------------------------------------------------------===//
 // Composite attribute kinds
 
 class DictionaryAttrBase<Pred condition, string summary> :
     Attr<condition, summary> {
   let storageType = [{ ::mlir::DictionaryAttr }];
+  let constBuilderCall = "$_builder.getDictionaryAttr($0)";
   let returnType = [{ ::mlir::DictionaryAttr }];
   let valueType = NoneType;
   let convertFromStorage = "$_self";
 }
 
 def DictionaryAttr
     : DictionaryAttrBase<CPred<"$_self.isa<::mlir::DictionaryAttr>()">,
                                "dictionary of named attribute values">;