flambda-backend: Fix assertion in caml_ephe_clean (#2192)

Author: mshinwell

runtime/caml/shared_heap.h

@@ -88,6 +88,10 @@ Caml_inline int is_marked(value v) {
   return Has_status_val(v, caml_global_heap_state.MARKED);
 }
 
+Caml_inline int is_not_markable(value v) {
+  return Has_status_val(v, NOT_MARKABLE);
+}
+
 void caml_redarken_pool(struct pool*, scanning_action, void*);
 
 intnat caml_sweep(struct caml_heap_state*, intnat);

runtime/weak.c

@@ -167,8 +167,9 @@ void caml_ephe_clean (value v) {
     else if (Is_block (child) && !Is_young (child)) {
       if (Tag_val (child) == Infix_tag) child -= Infix_offset_val (child);
       /* If we scanned all the keys and the data field remains filled,
-         then the mark phase must have marked it */
-      CAMLassert( is_marked (child) );
+         then the mark phase must have marked it (or alternatively the
+         value concerned is in static data etc). */
+      CAMLassert( is_marked (child) || is_not_markable (child) );
     }
 #endif
   }