clang: Only define OBJC_NEW_PROPERTIES when -x objective-c

Since 2009 (in r63846) we've been `#define`-ing OBJC_NEW_PROPERTIES all
the time on Darwin, but this macro only makes sense for `-x objective-c`
and `-x objective-c++`.  Restrict it to those cases (for which there is
already separate logic).

https://reviews.llvm.org/D72970
rdar://problem/10050342