[clang] [Clang] Extend lifetime of temporaries in mem-default-init for P2718R0 (PR #86960)

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yronglin wrote:

> LGTM But you need a release note to say that the implementation of P2718R0 is complete

Thanks for the review, added ReleaseNotes.

https://github.com/llvm/llvm-project/pull/86960