[libcxx] Improve tests to use the UNSUPPORTED lit directive

Quite a few libcxx tests seem to follow the format:
 #if _LIBCPP_STD_VER > X
   // Do test.
 #else
   // Empty test.
 #endif
We should instead use the UNSUPPORTED lit directive to exclude the test on
earlier C++ standards. This gives us a more accurate number of test passes
for those standards and avoids unnecessary conflicts with other lit
directives on the same tests.

Reviewers: bcraig, ericwf, mclow.lists

Differential revision: http://reviews.llvm.org/D20730

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@271108 91177308-0d34-0410-b5e6-96231b3b80d8

test/std/experimental/optional/optional.nullops/greater.pass.cpp

@@ -7,7 +7,7 @@
 //
 //===----------------------------------------------------------------------===//
 
-
+// UNSUPPORTED: c++98, c++03, c++11
 // <optional>
 
 // template <class T> constexpr bool operator>(const optional<T>& x, nullopt_t) noexcept;
@@ -17,7 +17,6 @@
 
 int main()
 {
-#if _LIBCPP_STD_VER > 11
     using std::experimental::optional;
     using std::experimental::nullopt_t;
     using std::experimental::nullopt;
@@ -37,5 +36,4 @@ int main()
     static_assert (noexcept(nullopt > o1), "");
     static_assert (noexcept(o1 > nullopt), "");
     }
-#endif
 }