[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.nullopt/nullopt_t.pass.cpp

@@ -7,6 +7,7 @@
 //
 //===----------------------------------------------------------------------===//
 
+// UNSUPPORTED: c++98, c++03, c++11
 // <optional>
 
 // struct nullopt_t{see below};
@@ -15,8 +16,6 @@
 #include <experimental/optional>
 #include <type_traits>
 
-#if _LIBCPP_STD_VER > 11
-
 using std::experimental::optional;
 using std::experimental::nullopt_t;
 using std::experimental::nullopt;
@@ -28,16 +27,12 @@ test(const nullopt_t&)
     return 3;
 }
 
-#endif
-
 int main()
 {
-#if _LIBCPP_STD_VER > 11
     static_assert((std::is_class<nullopt_t>::value), "");
     static_assert((std::is_empty<nullopt_t>::value), "");
     static_assert((std::is_literal_type<nullopt_t>::value), "");
     static_assert((!std::is_default_constructible<nullopt_t>::value), "");
     
     static_assert(test(nullopt) == 3, "");
-#endif
 }