[libcxx] [llvm] [libc++] LWG3870: Remove `voidify` (PR #110355)

[Louis Dionne via llvm-commits]

ldionne wrote:

Ack. Feel free to revert if you need more time. If you do that, you will run into a merge conflict with my modulemap patch (since it touched everything), but please don't revert that one. The conflict should be easy to get through by re-adding the `__memory/voidify.h` entry from the modulemap.

https://github.com/llvm/llvm-project/pull/110355