[libcxx] [test] Portability fix of std::ios_base::sync_with_stdio() test.

[amakc11]

Summary

The standard specifies effect of the std::ios_base::sync_with_stdio method only for false input value. Conformant implementations may leave the streams in unsynchronized state forever after calling this method with false value. So, all subsequent calls of this method will always return false even after calling this method with true. This patch marks extra checks as libc++ specific making the test more portable.

libcxx/test/std/input.output/iostreams.base/ios.base/ios.members.static/sync_with_stdio.pass.cpp

 
 #include "test_macros.h"
 
 int main(int, char**)
 {
     assert( std::ios_base::sync_with_stdio(false));
     assert(!std::ios_base::sync_with_stdio(false));
     assert(!std::ios_base::sync_with_stdio(true));
-    assert( std::ios_base::sync_with_stdio(true));
-    assert( std::ios_base::sync_with_stdio());
-    assert( std::ios_base::sync_with_stdio(false));
-    assert(!std::ios_base::sync_with_stdio());
-    assert( std::ios_base::sync_with_stdio());
+    LIBCPP_ASSERT( std::ios_base::sync_with_stdio(true));
+    LIBCPP_ASSERT( std::ios_base::sync_with_stdio());
+    LIBCPP_ASSERT( std::ios_base::sync_with_stdio(false));
+    LIBCPP_ASSERT(!std::ios_base::sync_with_stdio());
+    LIBCPP_ASSERT( std::ios_base::sync_with_stdio());
 
   return 0;
 }