Auto merge of #43904 - topecongiro:libsyntax/parse-attr, r=petrochenkov
Auto merge of #43904 - topecongiro:libsyntax/parse-attr, r=petrochenkov

Eat open paren when parsing list in libsyntax/parse/attr.rs

This PR adds a small refactoring:
```diff
      pub fn parse_meta_item_kind(&mut self) -> PResult<'a, ast::MetaItemKind> {
         Ok(if self.eat(&token::Eq) {
             ast::MetaItemKind::NameValue(self.parse_unsuffixed_lit()?)
-        } else if self.token == token::OpenDelim(token::Paren) {
+        } else if self.eat(&token::OpenDelim(token::Paren)) {
             ast::MetaItemKind::List(self.parse_meta_seq()?)
         } else {
-            self.eat(&token::OpenDelim(token::Paren));
             ast::MetaItemKind::Word
         })
     }
```
in `parse_meta_item_kind()`, the parser calls `self.eat(&token::OpenDelim(token::Paren));` before returning `ast::MetaItemKind::Word` just to add `(` to expected token. It seems more natural to eat the paren when parsing `ast::MetaItemKind::List`.

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