Rename uses of _ and __ because these are getting stepped on by macros from other system code.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@167038 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/include/iterator b/include/iterator
index 5747504..bcbf3ae 100644
--- a/include/iterator
+++ b/include/iterator
@@ -337,7 +337,7 @@
 struct __has_iterator_category
 {
 private:
-    struct __two {char _; char __;};
+    struct __two {char __lx; char __lxx;};
     template <class _Up> static __two __test(...);
     template <class _Up> static char __test(typename _Up::iterator_category* = 0);
 public: