Ada: Fix instantiation failure with qualified name of child generic unit

Ada: Fix instantiation failure with qualified name of child generic unit

This is again an issue with multiple levels of nested instances, and it
arises because the qualified name of the problematic child generic unit
is used (this works fine with the direct name), exposing the rather
questionable processing implemented for instances in Find_Expanded_Name.

The patch replaces this processing with the straightforward decoding of
the renaming scheme used in Sem_Ch12.

gcc/ada/
	PR ada/16214
	* sem_ch8.adb (Find_Expanded_Name): Consolidate and streamline the
	processing required for references to instances within themselves.

gcc/testsuite/
	* gnat.dg/specs/generic_inst6.ads: New test.
	* gnat.dg/specs/generic_inst6_pkg1-child.ads: New helper.
	* gnat.dg/specs/generic_inst6_pkg1-child-grand1.ads: Likewise.
	* gnat.dg/specs/generic_inst6_pkg1-child-grand2.ads: Likewise.
	* gnat.dg/specs/generic_inst6_pkg1.ads: Likewise.
	* gnat.dg/specs/generic_inst6_pkg2.ads: Likewise.
	* gnat.dg/specs/generic_inst6_pkg3.ads: Likewise.