Implement #[define_opaque] attribute for functions.

compiler/rustc_hir/src/stable_hash_impls.rs

@@ -106,7 +106,7 @@ impl<'tcx, HirCtx: crate::HashStableContext> HashStable<HirCtx> for AttributeMap
     fn hash_stable(&self, hcx: &mut HirCtx, hasher: &mut StableHasher) {
         // We ignore the `map` since it refers to information included in `opt_hash` which is
         // hashed in the collector and used for the crate hash.
-        let AttributeMap { opt_hash, map: _ } = *self;
+        let AttributeMap { opt_hash, define_opaque: _, map: _ } = *self;
         opt_hash.unwrap().hash_stable(hcx, hasher);
     }
 }