Fix "not all control paths return a value" warning on MSVC

llvm-svn: 274746

clang/lib/Basic/Targets.cpp

@@ -1804,6 +1804,8 @@ public:
           return "610";
         case CudaArch::SM_62:
           return "620";
+         default:
+           llvm_unreachable("unhandled CudaArch");
         }
       }();
       Builder.defineMacro("__CUDA_ARCH__", CUDAArchCode);