Add regression test for type_const with unit struct ctor under mGCA

Unit struct constructors used as the RHS of a `#[type_const]` associated
const used to ICE during normalization because they were lowered as
`Const::new_unevaluated` with a Ctor def_id. The compiler now properly
constructs a concrete ValTree value for const constructors.

Author: AprilNEA

tests/ui/const-generics/mgca/type-const-ctor-148953.rs

@@ -0,0 +1,33 @@
+//! Regression test for <https://github.com/rust-lang/rust/issues/148953>
+//!
+//! Unit struct constructors used as the RHS of a `#[type_const]` associated
+//! const used to ICE during normalization because they were lowered as
+//! `Const::new_unevaluated` with a Ctor def_id. Fixed by adding proper const
+//! constructor support that produces a concrete ValTree value instead.
+
+//@ check-pass
+
+#![feature(min_generic_const_args, adt_const_params)]
+#![expect(incomplete_features)]
+
+use std::marker::ConstParamTy;
+
+#[derive(ConstParamTy, PartialEq, Eq)]
+struct S;
+
+impl S {
+    #[type_const]
+    const N: S = S;
+}
+
+#[derive(ConstParamTy, PartialEq, Eq)]
+enum E {
+    V,
+}
+
+impl E {
+    #[type_const]
+    const M: E = { E::V };
+}
+
+fn main() {}