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_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 }
                                     });