Add reference link for HIR ID validator

Author: reddevilmidzy

src/hir/lowering.md

@@ -16,7 +16,7 @@ of such structures include but are not limited to
     * Converted to a virtual `existential type` declaration
 
 Lowering needs to uphold several invariants in order to not trigger the
-sanity checks in `compiler/rustc_passes/src/hir_id_validator.rs`:
+sanity checks in [`compiler/rustc_passes/src/hir_id_validator.rs`][hir_id_validator]:
 
 1. A `HirId` must be used if created. So if you use the `lower_node_id`,
   you *must* use the resulting `NodeId` or `HirId` (either is fine, since
@@ -33,6 +33,8 @@ sanity checks in `compiler/rustc_passes/src/hir_id_validator.rs`:
   which produces both a new `NodeId` as well as automatically lowering it
   for you so you also get the `HirId`.
 
+[hir_id_validator]: https://github.com/rust-lang/rust/blob/HEAD/compiler/rustc_passes/src/hir_id_validator.rs
+
 If you are creating new `DefId`s, since each `DefId` needs to have a
 corresponding `NodeId`, it is advisable to add these `NodeId`s to the
 `AST` so you don't have to generate new ones during lowering. This has