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

[Simon Pilgrim via cfe-commits]

RKSimon wrote:

Thanks @c8ef - are you OK to add the remaining integer reduction cases next? Also, please can you also update the LanguageExtensions docs page to mention that they are constexpr legal?

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