Trans enum & tuple struct constructors in place.

[luqmana]

Fixes #15400.

[pczarn]

@luqmana, could you elaborate? I tried implementing nested C-like enums. I lack knowledge on how to trans something like NestedCEnumVariant(FlatCEnumVariant) to a single store.

[luqmana]

@pczarn Right now when you have something like let x = Some(23i) we create a function Some that takes an int and returns Option<int> and we'd do this for any type we use with Option. This pull makes it such that we don't generate all these little functions and do those function calls. Instead what it does is just construct the Option in place.

[pczarn]

@luqmana Interesting. I see that I need an identity function that simply casts the argument. In addition to the representation check, of course.

Thanks.