Introduce {ConstantElementsPair} struct for type safety.

test/cctest/interpreter/bytecode_expectations/ArrayLiterals.golden

Index: test/cctest/interpreter/bytecode_expectations/ArrayLiterals.golden
diff --git a/test/cctest/interpreter/bytecode_expectations/ArrayLiterals.golden b/test/cctest/interpreter/bytecode_expectations/ArrayLiterals.golden
index a59ff9c213cbad14ab3abf288c0de87c09407386..2d57edf4a0614ecbfbebe57c0d02c73d2a918754 100644
--- a/test/cctest/interpreter/bytecode_expectations/ArrayLiterals.golden
+++ b/test/cctest/interpreter/bytecode_expectations/ArrayLiterals.golden
@@ -18,7 +18,7 @@ bytecodes: [
   /*   51 S> */ B(Return),
 ]
 constant pool: [
-  FIXED_ARRAY_TYPE,
+  CONSTANT_ELEMENTS_PAIR_TYPE,
 ]
 handlers: [
 ]
@@ -48,7 +48,7 @@ bytecodes: [
   /*   66 S> */ B(Return),
 ]
 constant pool: [
-  FIXED_ARRAY_TYPE,
+  CONSTANT_ELEMENTS_PAIR_TYPE,
 ]
 handlers: [
 ]
@@ -66,7 +66,7 @@ bytecodes: [
   /*   62 S> */ B(Return),
 ]
 constant pool: [
-  FIXED_ARRAY_TYPE,
+  CONSTANT_ELEMENTS_PAIR_TYPE,
 ]
 handlers: [
 ]
@@ -108,9 +108,9 @@ bytecodes: [
   /*   77 S> */ B(Return),
 ]
 constant pool: [
-  FIXED_ARRAY_TYPE,
-  FIXED_ARRAY_TYPE,
-  FIXED_ARRAY_TYPE,
+  CONSTANT_ELEMENTS_PAIR_TYPE,
+  CONSTANT_ELEMENTS_PAIR_TYPE,
+  CONSTANT_ELEMENTS_PAIR_TYPE,
 ]
 handlers: [
 ]