[Attributor] Generalize calloc handling in heap-to-stack for any init value [NFC]

Rewrite the calloc specific handling in heap-to-stack to allow arbitrary init values.  The basic problem being solved is that if an allocation is initilized to anything other than zero, this must be explicitly done for the formed alloca as well.

This covers the calloc case today, but once a couple of earlier guards are removed in this code, downstream allocators with other init values could also be handled.

Inspired by discussion on D116971