Silence an MSVC warning about not all control paths returning a value; NFC.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@239814 91177308-0d34-0410-b5e6-96231b3b80d8

lib/IR/LLVMContext.cpp

@@ -210,6 +210,7 @@ static const char *getDiagnosticMessagePrefix(DiagnosticSeverity Severity) {
   case DS_Note:
     return "note";
   }
+  llvm_unreachable("Unknown DiagnosticSeverity");
 }
 
 void LLVMContext::diagnose(const DiagnosticInfo &DI) {