[clang] [ABITest] Use _Alignof instead of __alignof__ for ABI alignment (PR #179031)

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mugiwaraluffy56 wrote:

> Yay!
> 
> This seems good to me.
> 
> It would be good for tests that were generated based on existing tests, including ABITestGen, to be redone, but this is probably fine to start.
> 
> The thing that brought the issue to my attention was clang/test/Sema/attr-riscv-rvv-vector-bits.c and similar tests that are essentially "generated test plus some modifications"... probably copied off another test that was the actual original generated one... and so on. They could be handled as a followup cleanup issue, though, as it is probably unrealistic to tackle all of them in one PR.

thanks! happy to look at the other tests in a follow up PR if needed.

https://github.com/llvm/llvm-project/pull/179031