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_span/src/symbol.rs

@@ -2083,6 +2083,7 @@ symbols! {
         type_ascribe,
         type_ascription,
         type_changing_struct_update,
+        type_const,
         type_id,
         type_ir_inherent,
         type_length_limit,