[libc++] Clean up pair's constructors and assignment operators

This patch makes std::pair's constructors and assignment operators
closer to conforming in C++23. The only missing bit I am aware of
now is `reference_constructs_from_temporary_v` checks, which we
don't have the tools for yet.

This patch also refactors a long-standing non-standard extension where
we'd provide constructors for tuple-like types in all standard modes. The
criteria for being a tuple-like type are different from pair-like types
as introduced recently in the standard, leading to a lot of complexity
when trying to implement recent papers that touch the pair constructors.

After this patch, the pre-C++23 extension is provided in a self-contained
block so that we can easily deprecate and eventually remove the extension
in future releases.

Differential Revision: https://reviews.llvm.org/D143914