[llvm] [ADT] Fix llvm::concat_iterator for `ValueT == common_base_class *` (PR #144744)

[Javier Lopez-Gomez via llvm-commits]

================
@@ -1032,13 +1032,17 @@ class concat_iterator
 
   static constexpr bool ReturnsByValue =
       !(std::is_reference_v<decltype(*std::declval<IterTs>())> && ...);
+  static constexpr bool ReturnsConvertiblePointer =
+      std::is_pointer_v<ValueT> &&
+      (std::is_convertible_v<decltype(*std::declval<IterTs>()), ValueT> && ...);
 
   using reference_type =
-      typename std::conditional_t<ReturnsByValue, ValueT, ValueT &>;
+      typename std::conditional_t<ReturnsByValue || ReturnsConvertiblePointer,
+                                  ValueT, ValueT &>;
 
-  using handle_type =
-      typename std::conditional_t<ReturnsByValue, std::optional<ValueT>,
-                                  ValueT *>;
+  using handle_type = typename std::conditional_t<
+      ReturnsConvertiblePointer, ValueT,
+      std::conditional_t<ReturnsByValue, std::optional<ValueT>, ValueT *>>;
----------------
jalopezg-git wrote:

I have split that into the types below and added a clarifying comment.  PTAL.
```c++
  using optional_value_type =
      std::conditional_t<ReturnsByValue, std::optional<ValueT>, ValueT *>;
  using handle_type = typename std::conditional_t<ReturnsConvertiblePointer,
                                                  ValueT, optional_value_type>;
```

https://github.com/llvm/llvm-project/pull/144744