CodeGen: add a LLVM_FALLTHROUGH to a fallthrough (NFC)

Drive by cleanup noticed while investigating an IR verifier assertion.

llvm-svn: 293867
diff --git a/clang/lib/CodeGen/CGClass.cpp b/clang/lib/CodeGen/CGClass.cpp
index 3152b6b..c3e907e 100644
--- a/clang/lib/CodeGen/CGClass.cpp
+++ b/clang/lib/CodeGen/CGClass.cpp
@@ -1416,9 +1416,7 @@
   // we'd introduce *two* handler blocks.  In the Microsoft ABI, we
   // always delegate because we might not have a definition in this TU.
   switch (DtorType) {
-  case Dtor_Comdat:
-    llvm_unreachable("not expecting a COMDAT");
-
+  case Dtor_Comdat: llvm_unreachable("not expecting a COMDAT");
   case Dtor_Deleting: llvm_unreachable("already handled deleting case");
 
   case Dtor_Complete:
@@ -1433,7 +1431,9 @@
                             /*Delegating=*/false, LoadCXXThisAddress());
       break;
     }
+
     // Fallthrough: act like we're in the base variant.
+    LLVM_FALLTHROUGH;
 
   case Dtor_Base:
     assert(Body);