Feature gate attributes starting with rustc_.

[nikomatsakis]

This can be considered a downpayment on rust-lang/rfcs#572.

r? @brson
cc @pnkfelix

[nikomatsakis]

cc @Manishearth

[Manishearth]

Shouldn't rustc_move_fragments be behind its own gate? That seems like a "feature" to me.

[brson]

Awesome. This will make me much happier about random rustc_ attributes if they are forever gated.

[brson]

@bors: r+ 7ca06ea

If some of the existing attributes are misclassified then that can be addressed in a followup.

[nikomatsakis]

@Manishearth I don't consider attributes like rustc_move_fragments, rustc_variance, or rustc_error to be features. We never intend for stable code to use them, they're just hacks to allow for better unit-testing (but very convenient, useful hacks!). The idea was that rustc_attrs is for attributes we never intend to stabilize -- anything that might eventually be user-exposed should get its own feature-gate, I agree.

[nikomatsakis]

@bors r- subsumed by #22364