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

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mahtohappy wrote:

@Fznamznon in non-template version 
```else if (!Constructor->isDependentContext() &&
               !Constructor->isDelegatingConstructor())``` this is the check that is performed and `isDependentContext` is true for templates, so I have separate condition for template versions. I'm doing the same thing as non-templated version just in different if condition. 

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