hir: Create hir::ConstArgKind enum

This will allow lowering const params to a dedicated enum variant, rather
than to an `AnonConst` that is later examined during `ty` lowering.

compiler/rustc_passes/src/hir_stats.rs

@@ -452,7 +452,7 @@ impl<'v> hir_visit::Visitor<'v> for StatCollector<'v> {
         match ga {
             hir::GenericArg::Lifetime(lt) => self.visit_lifetime(lt),
             hir::GenericArg::Type(ty) => self.visit_ty(ty),
-            hir::GenericArg::Const(ct) => self.visit_anon_const(&ct.value),
+            hir::GenericArg::Const(ct) => self.visit_const_arg(ct),
             hir::GenericArg::Infer(inf) => self.visit_infer(inf),
         }
     }