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

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.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@294107 91177308-0d34-0410-b5e6-96231b3b80d8

src/condition_variable.cpp

@@ -14,7 +14,6 @@
 #include "condition_variable"
 #include "thread"
 #include "system_error"
-#include "cassert"
 
 _LIBCPP_BEGIN_NAMESPACE_STD