[ADT] Delete unused llvm::pointer_union_detail::AssignableFrom

Noticed by Zhiwei Chen

Author: MaskRay

llvm/include/llvm/ADT/PointerUnion.h

@@ -93,13 +93,6 @@ namespace pointer_union_detail {
     static constexpr int NumLowBitsAvailable = lowBitsAvailable<PTs...>();
   };
 
-  /// Implement assignment in terms of construction.
-  template <typename Derived, typename T> struct AssignableFrom {
-    Derived &operator=(T t) {
-      return static_cast<Derived &>(*this) = Derived(t);
-    }
-  };
-
   template <typename Derived, typename ValTy, int I, typename ...Types>
   class PointerUnionMembers;