[libcxx-commits] [libcxx] [libc++][NFC] Remove uses of add_{const, cv, volatile} (PR #85635)

[A. Jiang via libcxx-commits]

frederick-vs-ja wrote:

> This makes sense to me. It seems so obvious that I am left wondering why these type traits were used in the first place, but I can't think of anything. So LGTM.

I guess that was partially because of that the standard wording (verbosely) says so ([[tuple.helper]](https://eel.is/c++draft/tuple.helper), [[depr.tuple]](https://timsong-cpp.github.io/cppwp/n4950/depr.tuple), [[associative.general]](https://eel.is/c++draft/associative.general)).

The rest were from eadece333f6112502046fad0f2c062d13a37079b (https://reviews.llvm.org/D96657) and 0f79772ecccd3340c84b45f00144c7a5d5395cc8 (the reason for using `add_*` is still unknown to me).

https://github.com/llvm/llvm-project/pull/85635