Handle `#[doc]` attribute properly

[CohenArthur]

• [ ] Fix locus of simple attributes

#[doc] // { dg-error "...doc.. cannot be an empty attribute" }
struct Foo;

• [ ] Handle LITERAL and META_ITEM properly
• [ ] Handle #[doc(alias(...))] instead of just #[doc(alias = ...)]
• [ ] Handle other doc attributes than alias (reference)