Ada: Fix segfault for instantiation on function call returning string

Ada: Fix segfault for instantiation on function call returning string

The problem is that a transient scope is created during the analysis of the
actual parameters of the instantiation and this discombobulates the complex
handling of scopes in Sem_Ch12.

gcc/ada/
	PR ada/78175
	* sem_ch12.adb (Hide_Current_Scope): Deal with a transient scope
	as current scope.
	(Remove_Parent): Likewise.

gcc/testsuite/
	* gnat.dg/generic_inst15.adb: New test.
	* gnat.dg/generic_inst15_pkg-g.ads: New helper.
	* gnat.dg/generic_inst15_pkg.ads: Likewise.