ensure that `'_` and GATs yields errors

[nikomatsakis]

It's not clear how '_ and GATs should interact. We should add tests that using '_ either in the parameter list of a GAT or in its value is prohibited.

e.g., both of these should error (and currently do)

#![feature(generic_associated_types)]

trait Foo {
    type Item<'a>;
}

fn foo(x: &impl Foo<Item<'_> = u32>) { }

#![feature(generic_associated_types)]

trait Foo {
    type Item<'a>;
}

fn foo(x: &impl for<'a> Foo<Item<'a> = &'_ u32>) { }

[marmeladema]

@bors claim