Show the variant name for univariant enums

Previously, only the fields would be displayed with no indication of the
variant name. If you already knew the enum was univariant, this was ok
but if the enum was univariant because of layout, for example, a
`Result<T, !>` then it could be very confusing which variant was the
active one.

compiler/rustc_codegen_ssa/src/debuginfo/type_names.rs

@@ -397,6 +397,17 @@ pub fn push_debuginfo_type_name<'tcx>(
             output.push_str("enum$<");
             push_item_name(tcx, def.did, true, output);
             push_generic_params_internal(tcx, substs, output, visited);
+
+            if let Variants::Single { index: variant_idx } = &layout.variants {
+                // Uninhabited enums can't be constructed and should never need to be visualized so
+                // skip this step for them.
+                if def.variants.len() != 0 {
+                    let variant = def.variants[*variant_idx].ident.as_str();
+
+                    output.push_str(&format!(", {}", variant));
+                }
+            }
+
             push_close_angle_bracket(tcx, output);
         }
     }