Rename _LIBCPP_DEBUG2 to _LIBCPP_DEBUG.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@189140 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/src/debug.cpp b/src/debug.cpp
index e840b22..d0e8679 100644
--- a/src/debug.cpp
+++ b/src/debug.cpp
@@ -7,7 +7,7 @@
 //
 //===----------------------------------------------------------------------===//
 
-#define _LIBCPP_DEBUG2 1
+#define _LIBCPP_DEBUG 1
 #include "__config"
 #include "__debug"
 #include "functional"