[clang] [libcxx] [llvm] Triple::normalize: Set OS for 3-component triple with none as middle (PR #89638)

[YunQiang Su via cfe-commits]

wzssyqa wrote:

@peterwaller-arm Ohh, there is so many `-none-unknown-` in current code. I guess it may be widely used.
Do we really want to change all of them?

https://github.com/llvm/llvm-project/pull/89638