Disallow function pointers to #[rustc_args_required_const]

This commit disallows acquiring a function pointer to functions tagged as
`#[rustc_args_required_const]`. This is intended to be used as future-proofing
for the stdsimd crate to avoid taking a function pointer to any intrinsic which
has a hard requirement that one of the arguments is a constant value.

src/test/compile-fail/rustc-args-required-const2.rs

@@ -0,0 +1,20 @@
+// Copyright 2018 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.
+
+#![feature(attr_literals, rustc_attrs, const_fn)]
+
+#[rustc_args_required_const(0)]
+fn foo(_a: i32) {
+}
+
+fn main() {
+    let a = foo; //~ ERROR: this function can only be invoked directly
+    a(2);
+}