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_lint/src/pass_by_value.rs

@@ -78,7 +78,7 @@ fn gen_args(cx: &LateContext<'_>, segment: &PathSegment<'_>) -> String {
                     .tcx
                     .sess
                     .source_map()
-                    .span_to_snippet(c.value.span)
+                    .span_to_snippet(c.span())
                     .unwrap_or_else(|_| "_".into()),
                 GenericArg::Infer(_) => String::from("_"),
             })