This commit primarily removes the `stability` field from `TyCtxt` as well as its
internal mutable state, instead using a query to build the stability index as
well as primarily using queries for other related lookups.
Like previous commits the calculation of the stability index is wrapped in a
`with_ignore` node to avoid regressing the current tests, and otherwise this
commit also introduces #44232 but somewhat intentionally so.
To accomplish this, we alter the checks in `rustc::middle::stability` to
use the `StabilityLevel` defined in `syntax::attr` (which includes the
version in which the feature was stabilized) rather than the local
`StabilityLevel` in the same module, and make the
`declared_stable_lang_features` field of
`syntax::feature_gate::Features` hold a Vec of feature-name, span
tuples (in analogy to the `declared_lib_features` field) rather than
just spans.
This is in the matter of issue #33394.
The 'stable_features' lint helps people progress from unstable to
stable Rust by telling them when they no longer need a `feature`
attribute because upstream Rust has declared it stable.
This compares to the existing 'unstable_features', which is used
to implement feature staging, and triggers on *any* use
of `#[feature]`.
