main_return_type.dd

`main` can now return type `noreturn` and supports return inference

If `main` never returns (due to an infinite loop or always throwing an
exception), it can now be declared as returning `noreturn`.
See https://dlang.org/spec/type.html#noreturn.

If `main` is declared with `auto`, the inferred return type must be
one of `void`, `int` and `noreturn`.