[libcxx] Never use <cassert> within libc++

Summary:
It is my opinion that libc++ should never use `<cassert>`, including in the `dylib`. This patch remove all uses of `assert` from within libc++ and replaces most of them with `_LIBCPP_ASSERT` instead.

Additionally this patch turn `LIBCXX_ENABLE_ASSERTIONS`  off by default, because the standard library should not be aborting user programs unless explicitly asked to.

Reviewers: mclow.lists, compnerd, smeenai

Reviewed By: mclow.lists

Subscribers: mgorny, cfe-commits

Differential Revision: https://reviews.llvm.org/D29063

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@292883 91177308-0d34-0410-b5e6-96231b3b80d8

include/__threading_support

@@ -25,7 +25,6 @@
 # include <pthread.h>
 # include <sched.h>
 #elif defined(_LIBCPP_HAS_THREAD_API_WIN32)
-#include <assert.h>
 #include <Windows.h>
 #include <process.h>
 #include <fibersapi.h>