Fix uninitialized memory use in ipa-modref

[Jan Hubicka]

> 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.

We can't because our vec does not accept non-pods and this needs to be
GGC safe since it points to trees.

Honza
> 
> Thanks,
> Martin