[libcxx][nfc] Remove <variant>'s dependence on <array>.

This will allow us to use variant in common_iterator. We do this by introducing a new `__light_array` type that variant uses instead of `std::array`.

Differential Revision: https://reviews.llvm.org/D105597

NOKEYCHECK=True
GitOrigin-RevId: 0849427faeab8d2e88868463496b1a8283e7dcda
2 files changed