Don't crash on non-existent path in constant.

The behavior here isn't really ideal, but we can't really do much better
given the current state of constant evaluation.

Fixes #28670, and probably a bunch of duplicates.

src/test/compile-fail/invalid-path-in-const.rs

@@ -0,0 +1,14 @@
+// Copyright 2015 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.
+
+fn main() {
+    fn f(a: [u8; u32::DOESNOTEXIST]) {}
+    //~^ ERROR unresolved path in constant expression
+}