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

llvm-svn: 239814
diff --git a/llvm/lib/IR/LLVMContext.cpp b/llvm/lib/IR/LLVMContext.cpp
index 0cd2a34..6d799e4 100644
--- a/llvm/lib/IR/LLVMContext.cpp
+++ b/llvm/lib/IR/LLVMContext.cpp
@@ -210,6 +210,7 @@
   case DS_Note:
     return "note";
   }
+  llvm_unreachable("Unknown DiagnosticSeverity");
 }
 
 void LLVMContext::diagnose(const DiagnosticInfo &DI) {