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

This reverts commit r292883. Unfortunately <string_view> uses
_LIBCPP_ASSERT in a way which is not compatible with the C++11 dylib
build. I'll investigate more tomorrow.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@292923 91177308-0d34-0410-b5e6-96231b3b80d8

src/condition_variable.cpp

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