Get tests running with warnings. Fix warnings in headers and tests

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@228344 91177308-0d34-0410-b5e6-96231b3b80d8

test/std/utilities/meta/meta.hel/integral_constant.pass.cpp

@@ -41,7 +41,9 @@ int main()
 
     std::false_type f1;
     std::false_type f2 = f1;
+    assert(!f2);
 
     std::true_type t1;
     std::true_type t2 = t1;
+    assert(t2);
 }