Improve suggestion for missing fmt str in println

Avoid using `concat!(fmt, "\n")` to improve the diagnostics being
emitted when the first `println!()` argument isn't a formatting string
literal.

src/test/ui/fmt/format-string-error.rs

@@ -10,8 +10,10 @@
 
 fn main() {
     println!("{");
+    //~^ ERROR invalid format string: expected `'}'` but string was terminated
     println!("{{}}");
     println!("}");
+    //~^ ERROR invalid format string: unmatched `}` found
     let _ = format!("{_foo}", _foo = 6usize);
     //~^ ERROR invalid format string: invalid argument name `_foo`
     let _ = format!("{_}", _ = 6usize);