[libc++] Don't manually override NDEBUG in the dylib build

LIBCXX_ENABLE_ASSERTIONS does not have any relationship to the `assert`
macro -- it only controls assertions that are internal to the library.
Playing around with `NDEBUG` only muddies the picture further than it
already is.

Also, remove a failing assertion in the benchmarks. That assertion had
never been exercised because we defined `NDEBUG` manually, and it was
failing since we introduced the ability to generate a benchmark vector
with the Quicksort adversary ordering (which is obviously not sorted).

This was split off of https://llvm.org/D121123.

Differential Revision: https://reviews.llvm.org/D121244