Rollup merge of #106946 - dtolnay:hashlinecolumn, r=m-ou-se

implement Hash for proc_macro::LineColumn

For use in `HashMap<LineColumn, TokenTree>` or `HashMap<LineColumn, Comment>`, for example.

[Here is an example of one case complicated by the absence of this impl.](https://github.com/andrewbaxter/genemichaels/blob/71bc45e417c3f9dae09f890f1ec4630e758e5c70/src/comments.rs#L25-L34)

Tracking issue: https://github.com/rust-lang/rust/issues/54725