Basic infrastructure for substitution tracking

[Jan Hubicka]

> There is no such restriction for DEBUG_STMTs in general.  Thus
> you could as well generate
> 
> # DEBUG aggr = { SR0.1, SR0.2 }
> 
> assuming that matches "debug constructor".

Which means that we will somehow bring this into RTL's CONCATs and track
it all the way through the RTL land?  It seems to me that RTL's CONCAT
is bit limited to represent this fully including i.e. bitfields, padding
and unions...

As for locations tracked by both aproaches after the dwarf2out fix, I
now get 16000 for current mainline and 17000 with the patch on tramp3d.
(this is without the SRA code). 

Honza
> 
> Richard.