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`.

clippy_lints/src/default.rs

@@ -198,7 +198,7 @@ impl LateLintPass<'_> for Default {
                 let ext_with_default = !variant
                     .fields
                     .iter()
-                    .all(|field| assigned_fields.iter().any(|(a, _)| a == &field.ident.name));
+                    .all(|field| assigned_fields.iter().any(|(a, _)| a == &field.name));
 
                 let field_list = assigned_fields
                     .into_iter()

clippy_lints/src/default_numeric_fallback.rs

@@ -161,7 +161,7 @@ impl<'a, 'tcx> Visitor<'tcx> for NumericFallbackVisitor<'a, 'tcx> {
                                 fields_def
                                     .iter()
                                     .find_map(|f_def| {
-                                        if f_def.ident == field.ident
+                                        if f_def.ident(self.cx.tcx) == field.ident
                                             { Some(self.cx.tcx.type_of(f_def.did)) }
                                         else { None }
                                     });

clippy_lints/src/inconsistent_struct_constructor.rs

@@ -76,7 +76,7 @@ impl LateLintPass<'_> for InconsistentStructConstructor {
             then {
                 let mut def_order_map = FxHashMap::default();
                 for (idx, field) in variant.fields.iter().enumerate() {
-                    def_order_map.insert(field.ident.name, idx);
+                    def_order_map.insert(field.name, idx);
                 }
 
                 if is_consistent_order(fields, &def_order_map) {

clippy_lints/src/matches.rs

@@ -1136,7 +1136,7 @@ fn check_wild_enum_match(cx: &LateContext<'_>, ex: &Expr<'_>, arms: &[Arm<'_>])
                 s.push_str("::");
                 s
             },
-            variant.ident.name,
+            variant.name,
             match variant.ctor_kind {
                 CtorKind::Fn if variant.fields.len() == 1 => "(_)",
                 CtorKind::Fn => "(..)",