Add test for cross-crate const ctor paths

Currently ICEs on this branch but should pass.

Author: camelid

tests/ui/const-generics/auxiliary/xcrate-const-ctor-a.rs

@@ -0,0 +1,6 @@
+#![feature(adt_const_params)]
+
+use std::marker::ConstParamTy;
+
+#[derive(Eq, PartialEq, ConstParamTy)]
+pub struct Foo;

tests/ui/const-generics/xcrate-const-ctor-b.rs

@@ -0,0 +1,15 @@
+//@ check-pass
+//@ aux-build:xcrate-const-ctor-a.rs
+
+#![feature(adt_const_params)]
+
+extern crate xcrate_const_ctor_a;
+use xcrate_const_ctor_a::Foo;
+
+fn bar<const N: Foo>() {}
+
+fn baz() {
+    bar::<{ Foo }>();
+}
+
+fn main() {}