Add universal_impl_trait feature gate

Move feature gate check to inside HIR lowering. Change error messages
and update tests.

src/test/compile-fail/impl-trait/feature-gate-universal.rs

@@ -0,0 +1,16 @@
+// Copyright 2017 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.
+
+// gate-test-universal_impl_trait
+
+fn foo(x: impl std::fmt::Debug) { print!("{:?}", x); }
+//~^ ERROR `impl Trait` in argument position is experimental
+
+fn main() {}

src/test/compile-fail/impl-trait/feature-gate.rs

@@ -11,6 +11,6 @@
 // gate-test-conservative_impl_trait
 
 fn foo() -> impl Fn() { || {} }
-//~^ ERROR `impl Trait` is experimental
+//~^ ERROR `impl Trait` in return position is experimental
 
 fn main() {}