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.

llvm-svn: 292923
diff --git a/libcxx/include/__config b/libcxx/include/__config
index a8a8d57..44f1344 100644
--- a/libcxx/include/__config
+++ b/libcxx/include/__config
@@ -824,9 +824,7 @@
 #   else
 #       error Supported values for _LIBCPP_DEBUG are 0 and 1
 #   endif
-# if !defined(_LIBCPP_BUILDING_LIBRARY)
 #   define _LIBCPP_EXTERN_TEMPLATE(...)
-# endif
 #endif
 
 #ifndef _LIBCPP_EXTERN_TEMPLATE
diff --git a/libcxx/include/__threading_support b/libcxx/include/__threading_support
index 35b6196..13ab769b 100644
--- a/libcxx/include/__threading_support
+++ b/libcxx/include/__threading_support
@@ -25,6 +25,7 @@
 # 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>