Disable -Wignored-attributes for now

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@364142 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 50ccbf6..dde769d 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -582,6 +582,7 @@
         -Wno-deprecated # FIXME: Remove this and fix all occurrences.
         -Wno-shift-sign-overflow # FIXME: Why do we need this with clang-cl but not clang?
         -Wno-double-promotion # FIXME: remove me
+        -Wno-ignored-attributes # FIXME: Caused by _LIBCPP_NODEBUG_TYPE not being supported on older clangs
       )
     endif()
 elseif("${CMAKE_CXX_COMPILER_ID}" MATCHES "GNU")