[ModRefInfo] Make enum ModRefInfo an enum class [NFC].

Summary:
Make enum ModRefInfo an enum class. Changes to ModRefInfo values should
be done using inline wrappers.
This should prevent future bit-wise opearations from being added, which can be more error-prone.

Reviewers: sanjoy, dberlin, hfinkel, george.burgess.iv

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D40933

llvm-svn: 320107
diff --git a/llvm/lib/Analysis/ObjCARCAliasAnalysis.cpp b/llvm/lib/Analysis/ObjCARCAliasAnalysis.cpp
index ed03406..096ea66 100644
--- a/llvm/lib/Analysis/ObjCARCAliasAnalysis.cpp
+++ b/llvm/lib/Analysis/ObjCARCAliasAnalysis.cpp
@@ -123,7 +123,7 @@
     // These functions don't access any memory visible to the compiler.
     // Note that this doesn't include objc_retainBlock, because it updates
     // pointers when it copies block data.
-    return MRI_NoModRef;
+    return ModRefInfo::NoModRef;
   default:
     break;
   }