[clang] [clang] constexpr built-in reduce `or` and `xor` function. (PR #116976)

[Simon Pilgrim via cfe-commits]

RKSimon wrote:

Please can you update the LanguageExtensions doc to mention that these are now constexpr https://github.com/llvm/llvm-project/blob/934140a3353f6d480a01a1f68d42899c926ee056/clang/docs/LanguageExtensions.rst?plain=1#L723-L765

https://github.com/llvm/llvm-project/pull/116976