Don't make a def id for impl_trait_in_bindings

tests/ui/impl-trait/in-bindings/dont-make-def-id.rs

@@ -0,0 +1,10 @@
+//@ check-pass
+
+// Make sure we don't create an opaque def id for ITIB.
+
+#![crate_type = "lib"]
+#![feature(impl_trait_in_bindings)]
+
+fn foo() {
+    let _: impl Sized = 0;
+}