[Ada] Enhance freezing code for instantiations

[Ada] Enhance freezing code for instantiations

gcc/ada/

	* sem_ch12.adb (Freeze_Package_Instance): Consistently consider
	the freeze node of the parent and use large inequality for
	Slocs.
	(Freeze_Subprogram_Instance): Likewise.
	(Insert_Freeze_Node_For_Instance): For an instance in a package
	spec with no source body that immediately follows, consider the
	body of the package for the placement of the freeze node and go
	to the outer level if there is no such body.