Improve recovery on malformed format call

If a comma in a format call is replaced with a similar token, then we
emit an error and continue parsing, instead of stopping at this point.

compiler/rustc_parse/src/parser/mod.rs

@@ -1233,6 +1233,10 @@ impl<'a> Parser<'a> {
                 *t == token::OpenDelim(token::Brace) || *t == token::BinOp(token::Star)
             })
     }
+
+    pub fn clear_expected_tokens(&mut self) {
+        self.expected_tokens.clear();
+    }
 }
 
 crate fn make_unclosed_delims_error(