WCA: We permit outer attributes that don't annotate anything at the end of where clauses

[fmease]

Note

Similar to #152820, this was found thanks to @matthiaskrgr fuzzing fmease/rasur versus rustc.
Downstream report: fmease/rasur#33.

We currently erroneously accept programs like the following:

#![feature(where_clause_attrs)]

struct S where #[allow()];
//             ~~~~~~~~~~ doesn't annotate anything; not present in the AST

#![feature(where_clause_attrs)]

fn f() where #[cfg(false)] {}
//           ~~~~~~~~~~~~~ doesn't annotate anything; not present in the AST

#![feature(where_clause_attrs)]

fn f<T>()
where
    T: Copy,
    #[cfg(true)]   // these two don't annotate anything;
    #[cfg(false)]  // they're absent from the AST
{}

---

Compare this with e.g.,

• fn f(#[allow()]) {} where we emit expected parameter name, found ) or
• struct X<#[allow()]>; which we reject with attribute without generic parameters or
• fn f() { #[allow()] } for which we report expected statement after outer attribute and so on.