Specify C++ standard version explicitly.

This is apparently required for the Mac tests.

Author: MForster

test/Interop/Cxx/operators/non-member-out-of-line.swift

@@ -1,5 +1,5 @@
 // RUN: %empty-directory(%t)
-// RUN: %target-clang -c %S/Inputs/non-member-out-of-line.cpp -I %S/Inputs -o %t/non-member-out-of-line.o
+// RUN: %target-clang -c %S/Inputs/non-member-out-of-line.cpp -I %S/Inputs -o %t/non-member-out-of-line.o -std=c++17
 // RUN: %target-build-swift %s -I %S/Inputs -o %t/non-member-out-of-line %t/non-member-out-of-line.o -Xfrontend -enable-cxx-interop
 // RUN: %target-codesign %t/non-member-out-of-line
 // RUN: %target-run %t/non-member-out-of-line