Properly encode AnonConst into crate metadata

Fixes #68104

Previous, we were encoding AnonConst as a regular Const, causing us to
treat them differently after being deserialized in another compilation
session.

src/test/ui/const-generics/issue-68104-print-stack-overflow.rs

@@ -0,0 +1,14 @@
+// aux-build:impl-const.rs
+// run-pass
+
+#![feature(const_generics)]
+#![allow(incomplete_features)]
+
+extern crate impl_const;
+
+use impl_const::*;
+
+pub fn main() {
+    let n = Num::<5>;
+    n.five();
+}