Fix macro redefinition warnings for _LIBCPP_NO_RTTI

llvm-svn: 302099
diff --git a/libcxx/include/__config b/libcxx/include/__config
index ee46860..598015c 100644
--- a/libcxx/include/__config
+++ b/libcxx/include/__config
@@ -314,7 +314,7 @@
 #define _LIBCPP_NO_EXCEPTIONS
 #endif
 
-#if !(__has_feature(cxx_rtti))
+#if !(__has_feature(cxx_rtti)) && !defined(_LIBCPP_NO_RTTI)
 #define _LIBCPP_NO_RTTI
 #endif