[libc++] Re-apply the switch-based std::variant implementation

This commit is a mass re-application of the following commits:

  7d15ece
  e0ec7a0
  02197f7
  a175a96

Those were temporarily reverted in 057028e, and never re-applied.
Re-committed by @ldionne (author edited for credit).

Differential Revision: https://reviews.llvm.org/D90168