While I'm here, resync a %select with the enum definition it selects on.

* nullptr used to be mapped to ERROR, now mapped to nullptr
* integral was missing
* expressions now have their own error message, so they won't reach
  this. Map them to ERROR.

Note that clang usually crashes before emitting this diagnostic anyway
(see PR13984), so this change alone doesn't have an observable effect.
It makes the code more correct though.

llvm-svn: 165095