review comments: modify note wording and change println

- Don't print the newline on its own to avoid the possibility of
  printing it out of order due to `stdout` locking.
- Modify wording of `concat!()` with non-literals to not mislead into
  believing that only `&str` literals are accepted.
- Add test for `concat!()` with non-literals.

src/libsyntax_ext/concat.rs

@@ -58,7 +58,7 @@ pub fn expand_syntax_ext(
     }
     if missing_literal.len() > 0 {
         let mut err = cx.struct_span_err(missing_literal, "expected a literal");
-        err.note("only `&str` literals can be passed to `concat!()`");
+        err.note("only literals (like `\"foo\"`, `42` and `3.14`) can be passed to `concat!()`");
         err.emit();
     }
     let sp = sp.apply_mark(cx.current_expansion.mark);