https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70512

this fixes the canonical type mismatch of 70512.  As surmised we don't correct 
the ptr-to and ref-to types created during the structure parse when 
late-applying the may_alias attribute.

This  patch  adjusts fixup_attribute_variants to walk the ptr-to and ref-to 
lists setting the TYPE_REF_CAN_ALIAS_ALL flag.  We also walk those lists for the 
main variant itself.

It wasn't clear to me whether decl_attributes (attribs.c) should mark the ptr-to 
and ref-to lists of the type having the may_alias attribute applied in place?

By construction, we'll adjust the appropriate canonical types, IIUC.

ok?

nathan