[libc++] Revert switch-based std::variant implementation again.

These changes cause substantial binary size increases for non-opt builds.
For example, the visit.pass.cpp test grows from 20k to 420k.

Further work will be done to re-land this patch without the size increases,
but that work is proving too tricky to fix forward.

This patch fully reverts:

* 35d22691

And it partially reverts:

* bb43a0cd

The latter of which added XFAIL's to new variant tests
because the new implementation needlessly makes non-throwing code
paths in variant invoke throwing code.

This means the reverted change also breaks source backwards compat
with code compiled on OS X targeting older system dylibs. There is no
need for this to be the case. We should fix it before recommitting.

Reviewed as:
https://reviews.llvm.org/D91662