Reimplement create_constant_zero in builder for Juno codegen

This implements the create_constant_zero which had previously existed in the Hercules IR builder but now without using the zero constant that has been removed. This also required changing the create_constant_array to take the extents to be dynamic constants (rather than just u32 but without this change there is actually no way to create a constant dynamically sized array which is necessary in many circumstances.

The create_constant_zero is used in a few places in Juno codegen and would require substantial rewrites to remove (and the rewrite would in essence do the exact same thing this function does just using the representation of types from the front-end).

requested review from @rarbore2

assigned to @aaronjc4

changed the description

Since the frontend relies on being able to do create_constant_zero, I think it's fine to have a wrapper as is implemented here (although it is definitely an artifact of how things were previously). As for lowering non-zero constant sized arrays, the only sane way I see to do this is to create a zero array constant and generate a write node per non-zero element at each necessary index. It does mean you can't map a non-zero constant array at the Juno level to a single constant at the Hercules IR level. Hopefully this is ok? @aaronjc4

approved this merge request

mentioned in commit bf1b9c90

merged

Yeah, for non-zero constant arrays we can just generate writes from the front-end. The change here was to allow zero constant arrays to have extents specified by non-constant values.