[libc++] Introduce __identity_t<T>. NFCI.
This is just a shorter synonym for `__identity<T>::type`.
Use it consistently throughout, where possible.
There is still some metaprogramming in <memory> and <variant>
where `__identity` is being used _without_ immediately calling
`::type` on it; but this is the unusual case, and it will become
even less usual as we start deliberately protecting certain types
against deduction (e.g. D97742).
Differential Revision: https://reviews.llvm.org/D97862
GitOrigin-RevId: 09fa1d0e50a3d907e8596f6ea49780b7187b727e
5 files changed
