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

[Jakub Kuderski via llvm-commits]

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@@ -1088,7 +1089,7 @@ class concat_iterator
     if (Begin == End)
       return {};
 
-    if constexpr (ReturnsByValue)
+    if constexpr (ReturnsValueOrPointer)
       return *Begin;
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kuhar wrote:

So this wraps pointers with `std::optional`? Why can't we return pointers here?

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