[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.literals/literal2.fail.cpp

@@ -7,14 +7,13 @@
 // Source Licenses. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++98, c++03, c++11
+
 #include <string>
 #include <cassert>
 
 int main()
 {
-#if _LIBCPP_STD_VER > 11 
     std::string foo  =   ""s;  // should fail w/conversion operator not found
-#else
-#error
-#endif
 }