[NFC][libc++][test] Enables variant test.

[Mordante]

Summary

Noticed this while working on D133326. Let's see whehter all compilers
now support this feature.

[philnik]

LGTM assuming CI passes.

[Mordante]

CI test

[Mordante]

Undo last change, was to the wrong diff.

libcxx/test/std/utilities/variant/variant.variant/variant.ctor/copy.pass.cpp

#endif // TEST_WORKAROUND_MSVC_BROKEN_IS_TRIVIALLY_COPYABLE
   static_assert(test_constexpr_copy_ctor_imp<2>(V(101)), "");
 }
 
 int main(int, char**) {
   test_copy_ctor_basic();
   test_copy_ctor_valueless_by_exception();
   test_copy_ctor_sfinae();
   test_constexpr_copy_ctor();
-#if 0
-// disable this for the moment; it fails on older compilers.
-//  Need to figure out which compilers will support it.
   { // This is the motivating example from P0739R0
     std::variant<int, double> v1(3);
     std::variant v2 = v1;
-  (void) v2;
+    (void)v2;
   }
-#endif
 
   return 0;
 }