AK/Complex: C++20-compatible comparison operators

Problem:
- Clang correctly reports non-`const` member function comparison
  operators as ambiguous.

Solution:
- Make them `const`.

AK/Complex.h

@@ -243,13 +243,13 @@ class [[gnu::packed]] Complex {
  }
 
  template<AK::Concepts::Arithmetic U>
- constexpr bool operator==(const Complex<U>& a)
+ constexpr bool operator==(const Complex<U>& a) const
  {
  return (this->real() == a.real()) && (this->imag() == a.imag());
  }
 
  template<AK::Concepts::Arithmetic U>
- constexpr bool operator!=(const Complex<U>& a)
+ constexpr bool operator!=(const Complex<U>& a) const
  {
  return !(*this == a);
  }

AK/Tests/TestRefPtr.cpp

@@ -119,7 +119,14 @@ TEST_CASE(assign_moved_self)
 {
  RefPtr<Object> object = adopt(*new Object);
  EXPECT_EQ(object->ref_count(), 1u);
+#ifdef __clang__
+# pragma clang diagnostic push
+# pragma clang diagnostic ignored "-Wself-move"
+#endif
  object = move(object);
+#ifdef __clang__
+# pragma clang diagnostic pop
+#endif
  EXPECT_EQ(object->ref_count(), 1u);
 }