[CIR][CodeGen] Adds union padding

[gitoleg]

This PR adds padding for union type, which is necessary in some cases (e.g. proper offset computation for an element of an array).

The previous discussion is here #1281

The idea is to add a notion about padding in the StructType in the same fashion as it's done for packed structures - as a bool argument in the constructor.

Now we can compute the proper union type size as a size of the largest element + size of padding type.

There are some downsides though - I had to add this padded word in many places. So take a look please!
There are many tests fixed and one new - union-padding

[bcardosolopes]

Overall looks good, you could go ahead and remove the lines you commented (and FIXME/TODOs that don't apply anymore).

Seems like even though it's adding the padded bit to regular structs (as updated testcases show), it's not using that information to do anything for them, is that true or did I miss something?

[gitoleg]

@bcardosolopes updated!

Seems like even though it's adding the padded bit to regular structs (as updated testcases show), it's not using that information to do anything for them, is that true or did I miss something?

It's true! Btw, maybe it will be good idea to use this padded for unions only? unless someone won't need something else? Because it looks a bit weird, especially for classes on my taste