Make doc comments cheaper with AttrKind.

`AttrKind` is a new type with two variants, `Normal` and `DocComment`. It's a
big performance win (over 10% in some cases) because `DocComment` lets doc
comments (which are common) be represented very cheaply.

`Attribute` gets some new helper methods to ease the transition:
- `has_name()`: check if the attribute name matches a single `Symbol`; for
  `DocComment` variants it succeeds if the symbol is `sym::doc`.
- `is_doc_comment()`: check if it has a `DocComment` kind.
- `{get,unwrap}_normal_item()`: extract the item from a `Normal` variant;
  panic otherwise.

Fixes #60935.

src/libsyntax/parse/parser/attr.rs

@@ -43,7 +43,7 @@ impl<'a> Parser<'a> {
                     just_parsed_doc_comment = false;
                 }
                 token::DocComment(s) => {
-                    let attr = attr::mk_sugared_doc_attr(s, self.token.span);
+                    let attr = attr::mk_doc_comment(s, self.token.span);
                     if attr.style != ast::AttrStyle::Outer {
                         let mut err = self.fatal("expected outer doc comment");
                         err.note("inner doc comments like this (starting with \
@@ -150,10 +150,9 @@ impl<'a> Parser<'a> {
         };
 
         Ok(ast::Attribute {
-            item,
+            kind: ast::AttrKind::Normal(item),
             id: attr::mk_attr_id(),
             style,
-            is_sugared_doc: false,
             span,
         })
     }
@@ -229,7 +228,7 @@ impl<'a> Parser<'a> {
                 }
                 token::DocComment(s) => {
                     // We need to get the position of this token before we bump.
-                    let attr = attr::mk_sugared_doc_attr(s, self.token.span);
+                    let attr = attr::mk_doc_comment(s, self.token.span);
                     if attr.style == ast::AttrStyle::Inner {
                         attrs.push(attr);
                         self.bump();