[libc++] ADL-proof <functional> by adding _VSTD:: qualification on calls.

- std::reference_wrapper
- std::function
- std::mem_fn

While I'm here, remove _VSTD:: qualification from calls to `declval`
because it takes no arguments and thus isn't susceptible to ADL.

Differential Revision: https://reviews.llvm.org/D92884

GitOrigin-RevId: 3c8e31e17b85c3d24ade9053d56c1998b4dd28cd
6 files changed