[Ada] Enhance freezing code for instantiations

[Pierre-Marie de Rodat]

This makes it possible for the freezing code to let the back-end
establish a proper order of elaboration of package and subprogram
instantiations in more cases, in particular with circularities, by
placing freeze nodes for them later in the expanded code in these cases.

Tested on x86_64-pc-linux-gnu, committed on trunk

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.
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