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

[Simon Pilgrim via cfe-commits]

https://github.com/RKSimon commented:

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.

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