[common] enforce correct CRTP usage for mix-in classes (#11880)

The mix-in helper classes like `Clearable<T>`, `Equatable<T>`, and
`Unequatable<T>` are intended for CRTP style inheritance, where `T`
is the derived class itself. A mistaken inheritance, such as `class
Foo : public Clearable<Bar>`, can compile successfully but lead to
subtle bugs.

This change enforces the correct CRTP usage at compile time. By making
the constructors of these helper classes `private` and declaring the
derived template class `T` as a `friend`, any incorrect inheritance
will now result in a build failure. This approach correctly detects
such a mistake, even if `Foo` and `Bar` happen to be `friend`s of
each other.

Additionally, `Equatable<T>` is updated to provide both `operator==`
and `operator!=`, removing its dependency on `Unequatable<T>`. This
change allows us to apply the `private` constructor enforcement to
`Equatable<T>` as well.

src/core/common/clearable.hpp

@@ -66,8 +66,13 @@ template <typename ObjectType> void ClearAllBytes(ObjectType &aObject)
  */
 template <typename Type> class Clearable
 {
+    friend Type;
+
 public:
     void Clear(void) { ClearAllBytes<Type>(*static_cast<Type *>(this)); }
+
+private:
+    Clearable(void) = default;
 };
 
 } // namespace ot

src/core/common/equatable.hpp

@@ -50,6 +50,8 @@ namespace ot {
  */
 template <typename Type> class Unequatable
 {
+    friend Type;
+
 public:
     /**
      * Overloads operator `!=` to evaluate whether or not two instances of `Type` are equal.
@@ -62,6 +64,9 @@ public:
      * @retval FALSE  If the two `Type` instances are equal.
      */
     bool operator!=(const Type &aOther) const { return !(*static_cast<const Type *>(this) == aOther); }
+
+private:
+    Unequatable(void) = default;
 };
 
 /**
@@ -72,8 +77,10 @@ public:
  * Users of this class should follow CRTP-style inheritance, i.e., the `Type` class itself should publicly inherit
  * from `Equatable<Type>`.
  */
-template <typename Type> class Equatable : public Unequatable<Type>
+template <typename Type> class Equatable
 {
+    friend Type;
+
 public:
     /**
      * Overloads operator `==` to evaluate whether or not two instances of `Type` are equal.
@@ -87,6 +94,19 @@ public:
     {
         return memcmp(static_cast<const Type *>(this), &aOther, sizeof(Type)) == 0;
     }
+
+    /**
+     * Overloads operator `!=` to evaluate whether or not two instances of `Type` are equal.
+     *
+     * @param[in]  aOther  The other `Type` instance to compare with.
+     *
+     * @retval TRUE   If the two `Type` instances are not equal.
+     * @retval FALSE  If the two `Type` instances are equal.
+     */
+    bool operator!=(const Type &aOther) const { return !(*static_cast<const Type *>(this) == aOther); }
+
+private:
+    Equatable(void) = default;
 };
 
 } // namespace ot