Use an empty TokenCursorFrame stack when capturing tokens

We will never need to pop  past our starting frame during token
capturing. Using an empty stack allows us to avoid pointless heap
allocations/deallocations.

compiler/rustc_parse/src/parser/mod.rs

@@ -1239,7 +1239,15 @@ impl<'a> Parser<'a> {
         f: impl FnOnce(&mut Self) -> PResult<'a, R>,
     ) -> PResult<'a, (R, Option<LazyTokenStream>)> {
         let start_token = (self.token.clone(), self.token_spacing);
-        let cursor_snapshot = self.token_cursor.clone();
+        let cursor_snapshot = TokenCursor {
+            frame: self.token_cursor.frame.clone(),
+            // We only ever capture tokens within our current frame,
+            // so we can just use an empty frame stack
+            stack: vec![],
+            desugar_doc_comments: self.token_cursor.desugar_doc_comments,
+            num_next_calls: self.token_cursor.num_next_calls,
+            append_unglued_token: self.token_cursor.append_unglued_token.clone(),
+        };
 
         let ret = f(self)?;