Migrate ast_lowering::lib and ast_lowering::item to SessionDiagnostic

compiler/rustc_error_messages/locales/en-US/ast_lowering.ftl

@@ -3,3 +3,16 @@ ast_lowering_generic_type_with_parentheses =
     .label = only `Fn` traits may use parentheses
 
 ast_lowering_use_angle_brackets = use angle brackets instead
+
+ast_lowering_invalid_abi =
+    invalid ABI: found `{$abi}`
+    .label = invalid ABI
+    .help = valid ABIs: {$valid_abis}
+
+ast_lowering_assoc_ty_parentheses =
+    parenthesized generic arguments cannot be used in associated type constraints
+
+ast_lowering_remove_parentheses = remove these parentheses
+
+ast_lowering_misplaced_impl_trait =
+    `impl Trait` only allowed in function and inherent method return types, not in {$position}