mlir.astnodes.Function.dump doesn't use the visibility attribute

We currently lose the visibility attribute of "private" when round tripping MLIR input text through PyMLIR in this example:
```
>>> import mlir
>>> input_string = "func private @resize_pointers(!llvm.ptr<i8>, index, index) -> ()"
>>> mlir.parse_string(input_string).body[0].dump()
'func @resize_pointers(!llvm.ptr< i8 >, index, index) {\n}'
>>> 
```
