[Ada] Fix assertion failure on derived private protected type

[Ada] Fix assertion failure on derived private protected type

This fixes an assertion failure on the instantiation of a generic
package on a type derived from the private view of a protected type,
ultimately caused by Finalize_Address returning Empty for the subtype
built for the generic actual type of the instantiation.

Finalize_Address has a special processing for untagged derivations of
private views, but it would no longer trigger for the subtype because
this subtype is now represented as a subtype of an implicit derived base
type instead of as the derived type of an implicit subtype previously.

2019-08-21  Eric Botcazou  <ebotcazou@adacore.com>

gcc/ada/

	* exp_util.adb (Finalize_Address): Deal consistently with
	subtypes of private protected types.

gcc/testsuite/

	* gnat.dg/prot9.adb, gnat.dg/prot9_gen.ads,
	gnat.dg/prot9_pkg1.ads, gnat.dg/prot9_pkg2.ads: New testcase.

From-SVN: r274778