The field is also renamed from `ident` to `name. In most cases,
we don't actually need the `Span`. A new `ident` method is added
to `VariantDef` and `FieldDef`, which constructs the full `Ident`
using `tcx.def_ident_span()`. This method is used in the cases
where we actually need an `Ident`.
This makes incremental compilation properly track changes
to the `Span`, without all of the invalidations caused by storing
a `Span` directly via an `Ident`.
* Implement `black_box` as intrinsic
Responsibility of implementing the black box is now lies on backend
* Remove some TODOs
* Update to nightly-2021-09-17
* CI: don't fail on warnings
