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/string b/include/string
index 89a00e5..00016ee 100644
--- a/include/string
+++ b/include/string
@@ -1094,14 +1094,14 @@
         union
         {
             unsigned char __size_;
-            value_type _;
+            value_type __lx;
         };
         value_type __data_[__min_cap];
     };
 
-    union _{__long _; __short __;};
+    union __lx{__long __lx; __short __lxx;};
 
-    enum {__n_words = sizeof(_) / sizeof(size_type)};
+    enum {__n_words = sizeof(__lx) / sizeof(size_type)};
 
     struct __raw
     {