[mlir][Complex] Add a convenience getValue() method.

[akuegel]

Summary

This method returns the value as std::complex<APFloat>

mlir/include/mlir/Dialect/Complex/IR/ComplexAttributes.td

AttrBuilderWithInferredContext<(ins "mlir::ComplexType":$type,
                         APFloat::rmNearestTiesToEven, &unused);
       APFloat imagFloat(imag);
       imagFloat.convert(elementType.getFloatSemantics(),
                         APFloat::rmNearestTiesToEven, &unused);
       return $_get(type.getContext(), realFloat, imagFloat, type);
     }]>
   ];
 
+  let extraClassDeclaration = [{
+    std::complex<APFloat> getValue() {
+      return std::complex<APFloat>(getReal(), getImag());
+    }
+  }];
+
   let genVerifyDecl = 1;
   let hasCustomAssemblyFormat = 1;
   let skipDefaultBuilders = 1;
 }
 
 #endif // COMPLEX_ATTRIBUTE