Adjust to_attr_token_stream.

It uses `once` chained with `(0..self.num_calls).map(...)` followed by
`.take(self.num_calls`. I found this hard to read. It's simpler to just
use `repeat_with`.

compiler/rustc_parse/src/parser/attr_wrapper.rs

@@ -106,7 +106,7 @@ impl ToAttrTokenStream for LazyAttrTokenStreamImpl {
         let mut cursor_snapshot = self.cursor_snapshot.clone();
         let tokens =
             std::iter::once((FlatToken::Token(self.start_token.0.clone()), self.start_token.1))
-                .chain((0..self.num_calls).map(|_| {
+                .chain(std::iter::repeat_with(|| {
                     let token = cursor_snapshot.next();
                     (FlatToken::Token(token.0), token.1)
                 }))