Replace __cplusplus comparisons and dialect __has_feature checks with TEST_STD_VER.

This is a huge cleanup that helps make the libc++ test suite more portable.
Patch from STL@microsoft.com. Thanks STL!


git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@272716 91177308-0d34-0410-b5e6-96231b3b80d8

test/std/containers/associative/set/iterator.pass.cpp

@@ -111,7 +111,7 @@ int main()
         for (int j = 1; j <= m.size(); ++j, ++i)
             assert(*i == j);
     }
-#if __cplusplus >= 201103L
+#if TEST_STD_VER >= 11
     {
         typedef int V;
         V ar[] =