[clang] [libcxx] [clang] Add builtin to clear padding bytes (prework for P0528R3) (PR #75371)

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huixie90 wrote:

> One new minor review comment.
> 
> Are there any other open remaining issues here? I'd like to get this finished and merged.

thanks a lot for the review. will address this tomorrow. reagarding the remaining issue, there is one open discussion
https://github.com/llvm/llvm-project/pull/75371#discussion_r2581679607
It was suggested to store the padding information during laying out the struct, instead of figuring out the padding in this builtin. there are pros and cons and I am suggesting to keep this PR's approach: figuring out the padding in this builtin

https://github.com/llvm/llvm-project/pull/75371