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`.

src/type_of.rs

@@ -57,7 +57,7 @@ pub fn uncached_gcc_type<'gcc, 'tcx>(cx: &CodegenCx<'gcc, 'tcx>, layout: TyAndLa
                 (layout.ty.kind(), &layout.variants)
             {
                 if def.is_enum() && !def.variants.is_empty() {
-                    write!(&mut name, "::{}", def.variants[index].ident).unwrap();
+                    write!(&mut name, "::{}", def.variants[index].name).unwrap();
                 }
             }
             if let (&ty::Generator(_, _, _), &Variants::Single { index }) =