Add fn signature query

crates/ra_analysis/src/db.rs

@@ -108,6 +108,7 @@ salsa::database_storage! {
             fn impls_in_module() for hir::db::ImplsInModuleQuery;
             fn body_hir() for hir::db::BodyHirQuery;
             fn body_syntax_mapping() for hir::db::BodySyntaxMappingQuery;
+            fn fn_signature() for hir::db::FnSignatureQuery;
         }
     }
 }