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

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

  7d15ece7
  e0ec7a02
  02197f7e
  a175a965

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

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