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

[ldionne]

Summary

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.

[• Quuxplusone]

LGTM if CI is green. But do I understand correctly that this is still two orthogonal patches — one related to LIBCXX_ENABLE_ASSERTIONS, and one completely orthogonal to that in the benchmark code? Prefer to land in two separate git commits, just for hygiene.

[ldionne]

In D121244#3368211, @Quuxplusone wrote:

LGTM if CI is green. But do I understand correctly that this is still two orthogonal patches — one related to LIBCXX_ENABLE_ASSERTIONS, and one completely orthogonal to that in the benchmark code? Prefer to land in two separate git commits, just for hygiene.

Nope -- those are related. Basically, when we stop defining -DNDEBUG, the assertion is not a noop anymore, and it fails (for the quicksort adversarial vector). Removing the manual setting of NDEBUG requires removing the assertion (which has been faulty for a while now, but we never noticed).

libcxx/CMakeLists.txt

if (APPLE)
     add_link_flags("-undefined dynamic_lookup")
   else()
     # Relax this restriction from HandleLLVMOptions
     string(REPLACE "-Wl,-z,defs" "" CMAKE_SHARED_LINKER_FLAGS "${CMAKE_SHARED_LINKER_FLAGS}")
   endif()
 endif()
 
 # Assertion flags =============================================================
-define_if(LIBCXX_ENABLE_ASSERTIONS -UNDEBUG)
-define_if_not(LIBCXX_ENABLE_ASSERTIONS -DNDEBUG)
 define_if(LIBCXX_ENABLE_ASSERTIONS -D_LIBCPP_DEBUG=0)
 define_if(LIBCXX_DEBUG_BUILD -D_DEBUG)
 if (LIBCXX_ENABLE_ASSERTIONS AND NOT LIBCXX_DEBUG_BUILD)
   # MSVC doesn't like _DEBUG on release builds. See PR 4379.
   define_if_not(LIBCXX_TARGETING_MSVC -D_DEBUG)
 endif()
 
 # Modules flags ===============================================================

libcxx/benchmarks/algorithms.bench.cpp

void fillValues(std::vector<std::string>& V, size_t N, Order O) {
   } else {
     while (V.size() < N)
       V.push_back(getRandomString(64));
   }
 }
 
 template <class T>
 void sortValues(T& V, Order O) {
-  assert(std::is_sorted(V.begin(), V.end()));
   switch (O) {
   case Order::Random: {
     std::random_device R;
     std::mt19937 M(R());
     std::shuffle(V.begin(), V.end(), M);
     break;
   }
   case Order::Ascending: