swift234

AST/SIL: Add a new Never type, and a TypeBase::isNever() check

Mostly NFC, this is just plumbing for the next patch.
Note that isNever() returns true for any uninhabited
enum.

It should be generalized so that stuff like (Never, Int)
is also known to be uninhabited, or even to support
generic substitutions that yield uninhabited types,
but for now I really see no reason to go that far, and
the current check for an enum with no cases seems
perfectly adequate.