rustc_span: Add `Symbol::to_ident_string` for use in diagnostic messages
rustc_span: Add `Symbol::to_ident_string` for use in diagnostic messages

aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,graydon/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust