Further macro protection by replacing _[A-Z] with _[A-Z]p

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@145410 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/include/__split_buffer b/include/__split_buffer
index 581f159..b63a6c7 100644
--- a/include/__split_buffer
+++ b/include/__split_buffer
@@ -392,8 +392,8 @@
         __first_ = __alloc_traits::allocate(__alloc(), __cap);
         __begin_ = __end_ = __first_;
         __end_cap() = __first_ + __cap;
-        typedef move_iterator<iterator> _I;
-        __construct_at_end(_I(__c.begin()), _I(__c.end()));
+        typedef move_iterator<iterator> _Ip;
+        __construct_at_end(_Ip(__c.begin()), _Ip(__c.end()));
     }
 }