Introduce a Never type for diverging expressions

[jjcnn]

The typechecker currently uses the Unknown type both as a placeholder for a not-yet-determined type, and to indicate the type of a (typechecked) diverging expression (e.g., return, break or continue). This conflation causes some convoluted IR generation code because the code generator cannot handle Unkonwn types.

Introducing a Never type to indicate diverging expressions would fix this problem, and thus allow a simplification of the IR code generation.

[ironcev]

As explained in #5562 we need the proper bottom type in general in all compilation phases.

Closing this issue in favor of #5562 which tracks the overall effort.