[llvm] [llvm] Support building with c++23 (PR #154372)

[Kyle Krüger via llvm-commits]

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@@ -36,7 +36,7 @@ class MachineFunctionAnalysis
     std::unique_ptr<MachineFunction> MF;
 
   public:
-    Result(std::unique_ptr<MachineFunction> MF) : MF(std::move(MF)) {}
+    Result(std::unique_ptr<MachineFunction> MF);
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kykrueger wrote:

Here is [another mention](https://github.com/hsutter/cppfront/pull/1083) of this issue from a different project. The consensus there is that the other compilers are allowing invalid c++ code according to the standard, and that . So rather I'd say that these errors are the result of a bug-fix.

As commented in the linked issue:

> If fact [cppreference.com](https://en.cppreference.com/w/cpp/memory/unique_ptr) states that:

>> std::unique_ptr may be constructed for an [incomplete type](https://en.cppreference.com/w/cpp/language/incomplete_type) T, such as to facilitate the use as a handle in the [pImpl idiom](https://en.cppreference.com/w/cpp/language/pimpl). If the default deleter is used, T must be complete at the point in code where the deleter is invoked, which happens in the destructor, move assignment operator, and reset member function of std::unique_ptr.

https://github.com/llvm/llvm-project/pull/154372