[Ada] Spurious error in instance nested in formal package

[Pierre-Marie de Rodat]

This patch removes a spurious error in a compilation of a generic body
GB that includes a formal package whose source GF includes a nested
generic package NGF, and GB includes instances of NGF and of further
generic units declared within NGF.

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

gcc/ada/

	* sem_ch12.adb (Load_Parent_Of_Generic): If an ancestor is an
	instance whose source appears within a formal package of the
	current unit, there is no body of the ancestor needed to
	complete the current generic compilation.
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