[clang] [clang] Avoid doing C++20 aggregate init during copy-initialization (PR #131320)

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offsetof wrote:

> I don't think this resolves the original issue.
> Part of the problem is that we are missing a diagnostic saying the implicit constructor is deleted (and why).

Fair enough; I linked that issue because the nonsensical diagnostic mentioned there ("no viable conversion from `S` to `int`") is a side-effect of the bug fixed by this PR. With the change applied, we get the usual "no matching constructor" with a list of candidates. But if we want something else, then the issue should remain open.


https://github.com/llvm/llvm-project/pull/131320