mgca: Lower all const paths as ConstArgKind::Path

When `#![feature(min_generic_const_args)]` is enabled, we now lower all
const paths in generic arg position to `hir::ConstArgKind::Path`. We
then lower assoc const paths to `ty::ConstKind::Unevaluated` since we
can no longer use the anon const expression lowering machinery. In the
process of implementing this, I factored out `hir_ty_lowering` code that
is now shared between lowering assoc types and assoc consts.

This PR also introduces a `#[type_const]` attribute for trait assoc
consts that are allowed as const args. However, we still need to
implement code to check that assoc const definitions satisfy
`#[type_const]` if present (basically is it a const path or a
monomorphic anon const).

compiler/rustc_feature/src/builtin_attrs.rs

@@ -576,6 +576,13 @@ pub static BUILTIN_ATTRIBUTES: &[BuiltinAttribute] = &[
         EncodeCrossCrate::Yes, experimental!(patchable_function_entry)
     ),
 
+    // Probably temporary component of min_generic_const_args.
+    // `#[type_const] const ASSOC: usize;`
+    gated!(
+        type_const, Normal, template!(Word), ErrorFollowing,
+        EncodeCrossCrate::Yes, min_generic_const_args, experimental!(type_const),
+    ),
+
     // ==========================================================================
     // Internal attributes: Stability, deprecation, and unsafe:
     // ==========================================================================