[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/strings/basic.string/string.iterators/iterators.pass.cpp

@@ -7,6 +7,8 @@
 //
 //===----------------------------------------------------------------------===//
 
+// UNSUPPORTED: c++98, c++03, c++11
+
 // <string>
 
 // iterator       begin();
@@ -21,7 +23,6 @@
 
 int main()
 {
-#if _LIBCPP_STD_VER > 11
     { // N3644 testing
         typedef std::string C;
         C::iterator ii1{}, ii2{};
@@ -69,5 +70,4 @@ int main()
         assert ( !(ii1 != ii2 ));
         assert ( !(ii1 != cii ));
     }
-#endif
 }