[clang] [Clang] Fix quadratic designated-initializer checking (PR #225573)

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AnonMiraj wrote:

Does the last commit work?
It is also a bit faster too.
On a side note, should I remove the else entirely, or maybe replace it with an assert? I kept it just to make sure it won't break anything, but I wasn't able to write any test that would force it to go into the else.

https://github.com/llvm/llvm-project/pull/225573