Add incremental test.

tests/incremental/const-generic-type-cycle.rs

@@ -0,0 +1,16 @@
+// Verify that we do not ICE when we try to overwrite an anon-const's type because of a trait
+// cycle.
+//
+// compile-flags: -Zincremental-ignore-spans
+// revisions: cpass cfail
+
+#![feature(trait_alias)]
+#![crate_type="lib"]
+
+#[cfg(cpass)]
+trait Bar<const N: usize> {}
+
+#[cfg(cfail)]
+trait Bar<const N: dyn BB> {}
+
+trait BB = Bar<{ 2 + 1 }>;