Rollup merge of #44262 - alexcrichton:repr-128-gate, r=nikomatsakis

rustc: Separately feature gate repr(i128)

Brought up during the discussion of #35118, the support for this is still
somewhat buggy and so stabilization likely wants to be considered independently
of the type itself.

src/test/compile-fail/feature-gate-repr128.rs

@@ -0,0 +1,17 @@
+// Copyright 2016 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+#[repr(u128)]
+enum A { //~ ERROR repr with 128-bit type is unstable
+    //~| HELP: add #![feature(repr128)]
+    A(u64)
+}
+
+fn main() {}