allow ?noreturn as a zero-bit type

[lolbinarycat]

The main usage of this would be for generics. It would allow "disabling" of certain parts of a generic type, for example a generic filesystem that has a Dir parameter could have that set to noreturn to represent a filesystem that did not include opening directories.

It would be a zero bit type because it only has one possible value, null.

Interestingly noreturn already works properly with error unions, with @sizeOf(anyerror) being equal to @sizeOf(anyerror!noreturn).

[zigazeljko]

Duplicate of #3257.