Fix diagnostic test to tolerate Clang diagnosing it as well.

Tuple has tests that ensure we diagnose non-lifetime extended
reference bindings inside tuples constructors. As of yesterday,
Clang now does this for us.

Adjust the test to tolerate the new diagnostics, while still
testing that we emit diagnostics of our own. Maybe after this
version of Clang has been adopted by most users we should
remove our diagnostics; but for now more error detection is
better!

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@337905 91177308-0d34-0410-b5e6-96231b3b80d8

test/libcxx/utilities/tuple/tuple.tuple/tuple.cnstr/PR20855_tuple_ref_binding_diagnostics.fail.cpp

@@ -46,7 +46,12 @@ void F(typename CannotDeduce<std::tuple<Args...>>::type const&) {}
 
 int main() {
 #if TEST_HAS_BUILTIN_IDENTIFIER(__reference_binds_to_temporary)
+  // Test that we emit our diagnostic from the library.
   // expected-error@tuple:* 8 {{"Attempted construction of reference element binds to a temporary whose lifetime has ended"}}
+
+  // Good news everybody! Clang now diagnoses this for us!
+  // expected-error@tuple:* 0+ {{reference member '__value_' binds to a temporary object whose lifetime would be shorter than the lifetime of the constructed object}}
+
   {
     F<int, const std::string&>(std::make_tuple(1, "abc")); // expected-note 1 {{requested here}}
   }