[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.object/optional.object.observe/dereference.pass.cpp

@@ -7,6 +7,7 @@
 //
 //===----------------------------------------------------------------------===//
 
+// UNSUPPORTED: c++98, c++03, c++11
 // <optional>
 
 // T& optional<T>::operator*();
@@ -19,8 +20,6 @@
 #include <type_traits>
 #include <cassert>
 
-#if _LIBCPP_STD_VER > 11
-
 using std::experimental::optional;
 
 struct X
@@ -29,11 +28,8 @@ struct X
     int test() {return 4;}
 };
 
-#endif  // _LIBCPP_STD_VER > 11
-
 int main()
 {
-#if _LIBCPP_STD_VER > 11
     {
         optional<X> opt(X{});
         assert((*opt).test() == 4);
@@ -45,5 +41,4 @@ int main()
         assert(false);
     }
 #endif  // _LIBCPP_DEBUG
-#endif  // _LIBCPP_STD_VER > 11
 }