[Ada] Fix incorrect freezing with generic child unit

[Pierre-Marie de Rodat]

The Analyze_Associations.Check_Generic_Parent function was using an
incorrect node as the instanciation node for the actual, possibly
leading to incorrect freeze node being created (and later crashing in
gigi). Using Get_Unit_Instantiation_Node fixes the issue.

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

gcc/ada/

	* sem_ch12.adb (Check_Generic_Parent): Use
	Get_Unit_Instantiation_Node instead of Next.
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