Save variant names in EnumData to reduce needless queries

We already have their names when anyway, and when in all (current)
situations where we're interested in an Enum's variants, we want
their names.

crates/ra_hir/src/code_model_api.rs

@@ -201,7 +201,7 @@ impl Enum {
         Ok(db.enum_data(self.def_id)?.name.clone())
     }
 
-    pub fn variants(&self, db: &impl HirDatabase) -> Cancelable<Option<Vec<EnumVariant>>> {
+    pub fn variants(&self, db: &impl HirDatabase) -> Cancelable<Vec<(Name, EnumVariant)>> {
         Ok(db.enum_data(self.def_id)?.variants.clone())
     }
 }