[clang] [Clang][Sema] Diagnosis for constexpr constructor not initializing a union member (PR #81225)

[Shafik Yaghmour via cfe-commits]

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@@ -2343,17 +2349,9 @@ static bool CheckConstexprFunctionBody(Sema &SemaRef, const FunctionDecl *Dcl,
     // - if the class is a union having variant members, exactly one of them
     //   shall be initialized;
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shafik wrote:

We need add a reference to [cwg2424](https://wg21.link/cwg2424) as well as a reference to [P1331r2](https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/p1331r2.pdf).

https://github.com/llvm/llvm-project/pull/81225