[code-utils] add OT_ARRAY_END() macro (#3722)

This commit adds `OT_ARRAY_END(aArray)` macro which returns a
pointer to the end of a given array (pointing to the past-the-end
element).

src/core/common/code_utils.hpp

@@ -49,6 +49,19 @@
  */
 #define OT_ARRAY_LENGTH(aArray) (sizeof(aArray) / sizeof(aArray[0]))
 
+/**
+ * This macro returns a pointer to end of a given array (pointing to the past-the-end element).
+ *
+ * Note that the past-the-end element is a theoretical element that would follow the last element in the array. It does
+ * not point to an actual element in array, and thus should not be dereferenced.
+ *
+ * @param[in]    Name of the array variable
+ *
+ * @returns Pointer to the past-the-end element.
+ *
+ */
+#define OT_ARRAY_END(aArray) (&aArray[OT_ARRAY_LENGTH(aArray)])
+
 /**
  * This macro returns a pointer aligned by @p aAlignment.
  *