[clang] [Clang] Implement P3074R7: trivial unions (C++26) (PR #185886)

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love1angel wrote:

@cor3ntin @ojhunt once again thanks for your review.

I address the issue here in the second commit:
https://github.com/llvm/llvm-project/pull/185886/changes/3faefdb97098efd7614510c1bb2ba9d642a86a79


thanks!

https://github.com/llvm/llvm-project/pull/185886