Introduce {ConstantElementsPair} struct for type safety.

test/cctest/interpreter/bytecode_expectations/ForIn.golden

Index: test/cctest/interpreter/bytecode_expectations/ForIn.golden
diff --git a/test/cctest/interpreter/bytecode_expectations/ForIn.golden b/test/cctest/interpreter/bytecode_expectations/ForIn.golden
index c8cbc7a61dfc46f174354e1fd8ce5c930e73a169..e03f83d3fc28fb03b964bbfc7b3e0c6c730a3733 100644
--- a/test/cctest/interpreter/bytecode_expectations/ForIn.golden
+++ b/test/cctest/interpreter/bytecode_expectations/ForIn.golden
@@ -132,7 +132,7 @@ bytecodes: [
   /*   80 S> */ B(Return),
 ]
 constant pool: [
-  FIXED_ARRAY_TYPE,
+  CONSTANT_ELEMENTS_PAIR_TYPE,
 ]
 handlers: [
 ]
@@ -188,7 +188,7 @@ bytecodes: [
 ]
 constant pool: [
   FIXED_ARRAY_TYPE,
-  FIXED_ARRAY_TYPE,
+  CONSTANT_ELEMENTS_PAIR_TYPE,
   ONE_BYTE_INTERNALIZED_STRING_TYPE ["a"],
 ]
 handlers: [
@@ -233,8 +233,8 @@ bytecodes: [
   /*   98 S> */ B(Return),
 ]
 constant pool: [
-  FIXED_ARRAY_TYPE,
-  FIXED_ARRAY_TYPE,
+  CONSTANT_ELEMENTS_PAIR_TYPE,
+  CONSTANT_ELEMENTS_PAIR_TYPE,
 ]
 handlers: [
 ]