Introduce {ConstantElementsPair} struct for type safety.

src/ast/compile-time-value.h

Index: src/ast/compile-time-value.h
diff --git a/src/ast/compile-time-value.h b/src/ast/compile-time-value.h
index 27351b79cc0b3784063b3757c46c5f26170e8c0c..d61443e5835e2708a9a82bfb88bdc3489ba21694 100644
--- a/src/ast/compile-time-value.h
+++ b/src/ast/compile-time-value.h
@@ -31,8 +31,8 @@ class CompileTimeValue : public AllStatic {
   // Get the type of a compile time value returned by GetValue().
   static LiteralType GetLiteralType(Handle<FixedArray> value);
 
-  // Get the elements array of a compile time value returned by GetValue().
-  static Handle<FixedArray> GetElements(Handle<FixedArray> value);
+  // Get the elements of a compile time value returned by GetValue().
+  static Handle<HeapObject> GetElements(Handle<FixedArray> value);
 
  private:
   static const int kLiteralTypeSlot = 0;