* Sema: fix atomic operand checking to allow pointers.
 * LLVM backend: implement pointer-like optional constants.
 * LLVM backend: fix `is_non_null` and `optional_payload` instructions
   to support pointer-like optionals.
 * Type: introduce `isPtrAtRuntime` method.
 * Type: fix `isPtrLikeOptional` to get the correct answer for allowzero
   pointers and slices.