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

[via cfe-commits]

mahtohappy wrote:

Inside that condition it doesn't check for if Union member is initialized but
`"constexpr constructor that does not initialize all members"` that's separate check.  

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