[clang] [Clang] Add `__builtin_reduce_addf` for ordered/unordered fp reductions (PR #176160)

[Gaëtan Bossu via cfe-commits]

https://github.com/gbossu commented:

The lowering seems sane to me, but I'm not familiar enough with pre-existing discussions to have a useful opinion. One thing I do wonder is whether it would be clearer to have names that immediately distinguish between ordered and unordered reductions. E.g.
 - __builtin_reduce_fadd_unordered`
 - __builtin_reduce_fadd_ordered`

https://github.com/llvm/llvm-project/pull/176160