more clippy::style fixes:

get_first
single_char_add_str
unnecessary_mut_passed
manual_map
manual_is_ascii_check

compiler/rustc_ast/src/util/comments.rs

@@ -62,7 +62,7 @@ pub fn beautify_doc_string(data: Symbol, kind: CommentKind) -> Symbol {
             CommentKind::Block => {
                 // Whatever happens, we skip the first line.
                 let mut i = lines
-                    .get(0)
+                    .first()
                     .map(|l| if l.trim_start().starts_with('*') { 0 } else { 1 })
                     .unwrap_or(0);
                 let mut j = lines.len();