Make explicit an unreachable NoDelim case in rustfmt.

Author: nnethercote

src/tools/rustfmt/src/macros.rs

@@ -562,7 +562,7 @@ fn delim_token_to_str(
                 ("{ ", " }")
             }
         }
-        DelimToken::NoDelim => ("", ""),
+        DelimToken::NoDelim => unreachable!(),
     };
     if use_multiple_lines {
         let indent_str = shape.indent.to_string_with_newline(context.config);