Put checking if anonct is a default into a method on hir map

compiler/rustc_hir/src/hir.rs

@@ -1422,6 +1422,9 @@ pub type Lit = Spanned<LitKind>;
 /// These are usually found nested inside types (e.g., array lengths)
 /// or expressions (e.g., repeat counts), and also used to define
 /// explicit discriminant values for enum variants.
+///
+/// You can check if this anon const is a default in a const param
+/// `const N: usize = { ... }` with [Map::opt_const_param_default_param_hir_id]
 #[derive(Copy, Clone, PartialEq, Eq, Encodable, Debug, HashStable_Generic)]
 pub struct AnonConst {
     pub hir_id: HirId,