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_infer/src/infer/error_reporting/mod.rs

@@ -1924,7 +1924,7 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
                 .fields
                 .iter()
                 .filter(|field| field.vis.is_accessible_from(field.did, self.tcx))
-                .map(|field| (field.ident.name, field.ty(self.tcx, expected_substs)))
+                .map(|field| (field.name, field.ty(self.tcx, expected_substs)))
                 .find(|(_, ty)| same_type_modulo_infer(ty, exp_found.found))
             {
                 if let ObligationCauseCode::Pattern { span: Some(span), .. } = *cause.code() {