Rollup merge of rust-lang#146094 - mohe2015:patch-2, r=lcnr

Make `Parser::parse_for_head` public for rustfmt usage

Similar to rust-lang#138511, I want to add [dioxus rsx](https://dioxuslabs.com/learn/0.6/reference/rsx) formatting to [my rustfmt fork](https://github.com/tucant/rustfmt) and it would be much easier if that method would be public. Thanks.

Author: GuillaumeGomez