`const trait Trait {}` syntax is accepted without a feature gate on 1.90 beta and master

[fmease]

const trait Trait {}

doesn't lead to a const_trait_impl feature gate error.

Affects 1.90 beta and nightly/master.

[fmease]

Oops, I thought the issue was present in 1.89 beta which would've been bad but it's all good.