Silence a warning from MSVC about not returning a value

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@188237 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Analysis/Consumed.cpp b/lib/Analysis/Consumed.cpp
index e556a8c..e64a26c 100644
--- a/lib/Analysis/Consumed.cpp
+++ b/lib/Analysis/Consumed.cpp
@@ -62,6 +62,7 @@
   case consumed::CS_Consumed:
     return "consumed";
   }
+  llvm_unreachable("invalid enum");
 }
 
 namespace {