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/cstddef b/include/cstddef
index 4b01be2..2467089 100644
--- a/include/cstddef
+++ b/include/cstddef
@@ -58,12 +58,12 @@
 
 struct _LIBCPP_VISIBLE nullptr_t
 {
-    void* _;
+    void* __lx;
 
     struct __nat {int __for_bool_;};
 
-    _LIBCPP_ALWAYS_INLINE _LIBCPP_CONSTEXPR nullptr_t() : _(0) {}
-    _LIBCPP_ALWAYS_INLINE _LIBCPP_CONSTEXPR nullptr_t(int __nat::*) : _(0) {}
+    _LIBCPP_ALWAYS_INLINE _LIBCPP_CONSTEXPR nullptr_t() : __lx(0) {}
+    _LIBCPP_ALWAYS_INLINE _LIBCPP_CONSTEXPR nullptr_t(int __nat::*) : __lx(0) {}
 
     _LIBCPP_ALWAYS_INLINE _LIBCPP_CONSTEXPR operator int __nat::*() const {return 0;}