Introduce {ConstantElementsPair} struct for type safety.

test/cctest/interpreter/bytecode_expectations/ForOf.golden

Index: test/cctest/interpreter/bytecode_expectations/ForOf.golden
diff --git a/test/cctest/interpreter/bytecode_expectations/ForOf.golden b/test/cctest/interpreter/bytecode_expectations/ForOf.golden
index 73f8a92126d3a035c1ae366be94ec998a378c6b1..7f1f8bda7b1bb1da7c65ebbeae6a942eff01cf6e 100644
--- a/test/cctest/interpreter/bytecode_expectations/ForOf.golden
+++ b/test/cctest/interpreter/bytecode_expectations/ForOf.golden
@@ -129,7 +129,7 @@ bytecodes: [
   /*   62 S> */ B(Return),
 ]
 constant pool: [
-  FIXED_ARRAY_TYPE,
+  CONSTANT_ELEMENTS_PAIR_TYPE,
   SYMBOL_TYPE,
   ONE_BYTE_INTERNALIZED_STRING_TYPE ["next"],
   ONE_BYTE_INTERNALIZED_STRING_TYPE ["done"],
@@ -432,7 +432,7 @@ bytecodes: [
   /*  113 S> */ B(Return),
 ]
 constant pool: [
-  FIXED_ARRAY_TYPE,
+  CONSTANT_ELEMENTS_PAIR_TYPE,
   SYMBOL_TYPE,
   ONE_BYTE_INTERNALIZED_STRING_TYPE ["next"],
   ONE_BYTE_INTERNALIZED_STRING_TYPE ["done"],
@@ -586,7 +586,7 @@ bytecodes: [
 ]
 constant pool: [
   FIXED_ARRAY_TYPE,
-  FIXED_ARRAY_TYPE,
+  CONSTANT_ELEMENTS_PAIR_TYPE,
   SYMBOL_TYPE,
   ONE_BYTE_INTERNALIZED_STRING_TYPE ["next"],
   ONE_BYTE_INTERNALIZED_STRING_TYPE ["done"],