AK: Add Retained<T>, like RetainPtr, but never null.

Also use some Clang attribute wizardry to get a warning for use-after-move.

Kernel/FIFO.cpp

@@ -3,7 +3,7 @@
 
 //#define FIFO_DEBUG
 
-RetainPtr<FIFO> FIFO::create()
+Retained<FIFO> FIFO::create()
 {
     return adopt(*new FIFO);
 }