Fix "control reaches end of non-void function" warning. NFCI.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@366856 91177308-0d34-0410-b5e6-96231b3b80d8

lib/IR/RemarkStreamer.cpp

@@ -118,6 +118,7 @@ formatToSerializer(remarks::Format RemarksFormat, raw_ostream &OS) {
   case remarks::Format::YAMLStrTab:
     return llvm::make_unique<remarks::YAMLStrTabSerializer>(OS);
   };
+  llvm_unreachable("Unknown remarks::Format enum");
 }
 
 Expected<std::unique_ptr<ToolOutputFile>>