[libcxx] [test] Avoid Clang's -Wunused-const-variable in is_constructible.pass.cpp.

This happens when using Clang with MSVC's STL, so there are no actual uses of this variable.

Fixes D31966.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@300079 91177308-0d34-0410-b5e6-96231b3b80d8

test/std/utilities/meta/meta.unary/meta.unary.prop/is_constructible.pass.cpp

@@ -251,6 +251,7 @@ int main()
     LIBCPP_STATIC_ASSERT(
         clang_disallows_valid_static_cast_bug !=
         std::__libcpp_is_constructible<int&&, ExplicitTo<int&&>>::value, "");
+    ((void)clang_disallows_valid_static_cast_bug); // Prevent unused warning
 #else
     static_assert(clang_disallows_valid_static_cast_bug == false, "");
     LIBCPP_STATIC_ASSERT(std::__libcpp_is_constructible<int&&, ExplicitTo<int&&>>::value, "");