Publicly expose (document) the type attribute of Constants

`ir.Constant` objects are constructed with an `ir.Type` argument, which is stored in the undocumented `self.type` attribute. That attribute is helpful for using `ir.Constant`s in places where an explicit type is needed, such as `alloca`:

```python
constant = get_some_constant()
slot = builder.alloca(constant.type)
builder.store(constant, slot)
```

But I don't feel good about using it unless it's clearly documented as part of the public API.

Start by locating the public API documentation or source entry point for ir.Constant and inspect how its constructor and attributes are presented. Document the existing type attribute as public, including its use with alloca, and consider the issue complete when users can find that documentation.

Written by the indexing model from the issue text.