Introduce {ConstantElementsPair} struct for type safety.

src/ast/compile-time-value.cc

Index: src/ast/compile-time-value.cc
diff --git a/src/ast/compile-time-value.cc b/src/ast/compile-time-value.cc
index eda536b7167e052ce9ee9adac50b26c2a70e6280..27dd29fee0a75af04e951f4f35e68fb1c46c63b2 100644
--- a/src/ast/compile-time-value.cc
+++ b/src/ast/compile-time-value.cc
@@ -48,8 +48,8 @@ CompileTimeValue::LiteralType CompileTimeValue::GetLiteralType(
   return static_cast<LiteralType>(literal_type->value());
 }
 
-Handle<FixedArray> CompileTimeValue::GetElements(Handle<FixedArray> value) {
-  return Handle<FixedArray>(FixedArray::cast(value->get(kElementsSlot)));
+Handle<HeapObject> CompileTimeValue::GetElements(Handle<FixedArray> value) {
+  return Handle<HeapObject>(HeapObject::cast(value->get(kElementsSlot)));
 }
 
 }  // namespace internal