Store a Symbol instead of an Ident in VariantDef/FieldDef

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`.

compiler/rustc_mir_dataflow/src/elaborate_drops.rs

@@ -491,7 +491,7 @@ where
             if let Some(variant_path) = subpath {
                 let base_place = tcx.mk_place_elem(
                     self.place,
-                    ProjectionElem::Downcast(Some(variant.ident.name), variant_index),
+                    ProjectionElem::Downcast(Some(variant.name), variant_index),
                 );
                 let fields = self.move_paths_for_fields(base_place, variant_path, &variant, substs);
                 values.push(discr.val);