Placate gcc's -Wreturn-type

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@149104 91177308-0d34-0410-b5e6-96231b3b80d8

lib/CodeGen/CGDecl.cpp

@@ -1166,6 +1166,7 @@ CodeGenFunction::getDestroyer(QualType::DestructionKind kind) {
   case QualType::DK_objc_weak_lifetime:
     return destroyARCWeak;
   }
+  llvm_unreachable("Unknown DestructionKind");
 }
 
 /// pushDestroy - Push the standard destructor for the given type.