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/debuginfo/mod.rs

@@ -174,7 +174,7 @@ fn dwarf_ty(&mut self, ty: Ty<'tcx>) -> UnitEntryId {
 
                     field_entry.set(
                         gimli::DW_AT_name,
-                        AttributeValue::String(field_def.ident.as_str().to_string().into_bytes()),
+                        AttributeValue::String(field_def.name.as_str().to_string().into_bytes()),
                     );
                     field_entry.set(
                         gimli::DW_AT_data_member_location,