[libcxx] Prefer UNSUPPORTED over conditional compilation for tests

[rmaprath]

Summary

We seem to have quite a few tests where an entire test is conditionalized over some range of _LIBCPP_STD_VER. We should consider getting rid of these and using the UNSUPPORTED lit directive instead.

This is mostly cosmetic, but sometimes can create problems if there are also XFAIL conditions on the same test. The test will be reported as an unexpected-pass when run with a std= option not compatible with the _LIBCPP_STD_VER check (because the test is trivially empty, it will pass rather than XFAIL).

I've attached a sample patch for one of the cases. If this looks OK, I can commit the whole hog.

[bcraig]

LGTM. Add @mclow.lists as a reviewer.

[mclow.lists]

LGTM.

test/libcxx/experimental/containers/sequences/dynarray/dynarray.overview/at.pass.cpp

 //
 //===----------------------------------------------------------------------===//
 
+// UNSUPPORTED: c++98, c++03, c++11
 // XFAIL: libcpp-no-exceptions
 // dynarray.overview
 

   
 #include <__config>
 
-#if _LIBCPP_STD_VER > 11
-
 #include <experimental/dynarray>
 #include <cassert>
 

     test<std::complex<double>> ( {} );
     test<std::string> ( {} );
 }
-#else
-int main() {}
-#endif