When encountering an undefined named lifetime, point to where it can be

This doesn't mention that using an existing lifetime is possible, but
that would hopefully be clear as always being an option. The intention
of this is to teach newcomers what the lifetime syntax is.

src/test/ui/parser/trait-object-trait-parens.stderr

@@ -33,6 +33,9 @@ LL |     let _: Box<(for<'a> Trait<'a>) + (Copy) + (?Sized)>;
 error[E0261]: use of undeclared lifetime name `'a`
   --> $DIR/trait-object-trait-parens.rs:11:31
    |
+LL | fn main() {
+   |        - help: consider introducing lifetime `'a` here: `<'a>`
+...
 LL |     let _: Box<(for<'a> Trait<'a>) + (Copy) + (?Sized)>;
    |                               ^^ undeclared lifetime