[clang][NFC] Add a test for CWG2254 to is_pointer_interconvertible_base_of tests

Resolution of that issue makes _any_ base class subobject interconvertible with the containing object, not just the first one.

Author: Endilll

clang/test/SemaCXX/type-traits.cpp

@@ -1864,6 +1864,7 @@ void is_pointer_interconvertible_base_of(int n)
   static_assert(!__is_pointer_interconvertible_base_of(Base2, Derived));
   static_assert(__is_pointer_interconvertible_base_of(Base, DerivedIndirect));
   static_assert(__is_pointer_interconvertible_base_of(Base, DerivedMultiple));
+  static_assert(__is_pointer_interconvertible_base_of(Base2, DerivedMultiple));
   static_assert(!__is_pointer_interconvertible_base_of(Base3, DerivedMultiple));
   static_assert(!__is_pointer_interconvertible_base_of(Base, DerivedAmbiguous));
   static_assert(__is_pointer_interconvertible_base_of(Base, DerivedPrivate));