[clang] [clang] constexpr built-in reduce add function. (PR #116243)

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c8ef wrote:

> This seems reasonable to me - I'd recommend you take a look at the min/max reductions to see if signed/unsigned can be correctly handled with this approach.

Thank you for your suggestions! Signed integer overflow in C++ is undefined behavior and cannot occur in a constexpr context. I have addressed the issue in the latest revision.

https://github.com/llvm/llvm-project/pull/116243