Fix uninitialized memory use in ipa-modref

[Martin Liška]

On 11/5/20 3:27 PM, Jan Hubicka wrote:
>     poly_int64 offset;
>     struct modref_parm_map parm_map;
>   
> +  parm_map.parm_offset_known = false;
> +  parm_map.parm_offset = 0;
> +

I'm curious, can't we use a proper C++ class construction.
The IPA pass is new and so we can make it more C++-ish? Similarly
for all newly introduced structs in mod ref.

Thanks,
Martin