fix test for older clang versions

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@361594 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/test/std/language.support/support.dynamic/destroying_delete_t.pass.cpp b/test/std/language.support/support.dynamic/destroying_delete_t.pass.cpp
index a2c9b8b..2ca6d19 100644
--- a/test/std/language.support/support.dynamic/destroying_delete_t.pass.cpp
+++ b/test/std/language.support/support.dynamic/destroying_delete_t.pass.cpp
@@ -15,6 +15,7 @@
 // UNSUPPORTED: c++98, c++03, c++11, c++14, c++17
 
 // UNSUPPORTED: apple-clang-9, apple-clang-10
+// UNSUPPORTED: clang-6
 
 #include <new>