Refine r251469 to give better (and more localizable) diagnostics

for all the reasons that ARC makes things implicitly unavailable.

llvm-svn: 251496

clang/lib/Sema/SemaExprObjC.cpp

@@ -3472,7 +3472,7 @@ diagnoseObjCARCConversion(Sema &S, SourceRange castRange,
     (castRange.isValid() ? castRange.getBegin() : castExpr->getExprLoc());
   
   if (S.makeUnavailableInSystemHeader(loc,
-                "converts between Objective-C and C pointers in -fobjc-arc"))
+                                 UnavailableAttr::IR_ARCForbiddenConversion))
     return;
 
   QualType castExprType = castExpr->getType();