Fix MSVC "not all control paths return a value" warning. NFC.

clang/lib/CodeGen/CGClass.cpp

diff --git a/clang/lib/CodeGen/CGClass.cpp b/clang/lib/CodeGen/CGClass.cpp
index 652622a..6beb2bc 100644
--- a/clang/lib/CodeGen/CGClass.cpp
+++ b/clang/lib/CodeGen/CGClass.cpp
@@ -2805,6 +2805,7 @@
   case CodeGenFunction::CFITCK_VMFCall:
     llvm_unreachable("unexpected sanitizer kind");
   }
+  llvm_unreachable("Unknown CFITypeCheckKind enum");
 }
 
 void CodeGenFunction::EmitVTablePtrCheckForCall(const CXXRecordDecl *RD,