rustc-ap-rustc_attr 646.0.0

Automatically published version of the package `rustc_attr` in the rust-lang/rust repository from commit 592e9c37008c2389451d28874a748f5b38612ca5 The publishing script for this crate lives at:

Functions and types dealing with attributes and meta items.

FIXME(Centril): For now being, much of the logic is still in rustc_ast::attr. The goal is to move the definition of MetaItem and things that don't need to be in syntax to this crate.