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/__mutex_base b/include/__mutex_base
index 538e89b..e936ad3 100644
--- a/include/__mutex_base
+++ b/include/__mutex_base
@@ -287,14 +287,14 @@
 
 struct _LIBCPP_VISIBLE cv_status
 {
-    enum _ {
+    enum __lx {
         no_timeout,
         timeout
     };
 
-    _ __v_;
+    __lx __v_;
 
-    _LIBCPP_INLINE_VISIBILITY cv_status(_ __v) : __v_(__v) {}
+    _LIBCPP_INLINE_VISIBILITY cv_status(__lx __v) : __v_(__v) {}
     _LIBCPP_INLINE_VISIBILITY operator int() const {return __v_;}
 
 };