Rollup merge of #33973 - zackmdavis:stable_features_warning_notes_version_stabilized, r=brson

stable features lint warning mentions version stabilized

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.

Fixes #33394.

![stable_features_version_lint_before_and_after](https://cloud.githubusercontent.com/assets/1076988/15659237/5d952a3a-267c-11e6-9181-c9e612eefd7d.png)

r? @brson (tagging Brian because he [wrote](https://github.com/rust-lang/rust/pull/21958) the lint)

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