[cxx-interop] Don't include unicode/ucal in C++ mode.

This is exported as a C header but in C++ mode ucal.h will expose
templates that aren't valid.

Author: zoecarver

CoreFoundation/Locale.subproj/CFCalendar_Internal.h

@@ -19,7 +19,7 @@
 #include "CFDateComponents.h"
 #include "CFDateInterval.h"
 
-#if __has_include(<unicode/ucal.h>)
+#if __has_include(<unicode/ucal.h>) && !defined(__cplusplus)
 #include <unicode/ucal.h>
 #else
 typedef void *UCalendar;