ada: Implement proper upcasting in more cases

ada: Implement proper upcasting in more cases

Upcasting (conversion from a tagged type extension to one of its parents)
is represented as a simple N_Type_Conversion node in the expanded code,
but translating it into a VIEW_CONVERT_EXPR is a bit problematic because
source and target types of the GCC node are supposed to have the same size
(at least in "non-pathological" cases).

That's why Gigi attempts to build an explicit chain of references to the
appropriate _Parent (sub)component instead, but it currently does that
only for simple (i.e. non-discriminated) tagged types.  This can be easily
extended to discriminated tagged types in not-too-dynamic cases (an example
is the ACATS c391002 test).

gcc/ada/ChangeLog:

	* gcc-interface/utils.cc (convert): Also extract the _Parent field
	to implement upcasting in the case where only the sizes match.