math_opt: Fix MSVC compile of attr_key.h

Author: Mizux

ortools/math_opt/elemental/attr_key.h

@@ -99,8 +99,20 @@ class AttrKey {
   }
 
   // Element iteration.
-  constexpr const value_type* begin() const { return element_ids_.begin(); }
-  constexpr const value_type* end() const { return element_ids_.end(); }
+  constexpr const value_type* begin() const {
+#if defined(_MSC_VER)
+	  return &(*element_ids_.begin());
+#else
+	  return element_ids_.begin();
+#endif
+  }
+  constexpr const value_type* end() const {
+#if defined(_MSC_VER)
+	  return &(*element_ids_.end());
+#else
+	  return element_ids_.end();
+#endif
+  }
 
   // `AttrKey` is comparable (ordering is lexicographic) and hashable.
   //