ada: Cleanup detection of code within generic instances

ada: Cleanup detection of code within generic instances

To check if a node is located in a generic instance we can either look
at Instantiation_Location or at the Instantiation_Depth, but just
looking at the location is simpler and more efficient.

Cleanup related to improved detection of references to uninitialized
objects; semantics is unaffected.

gcc/ada/

	* sem_ch13.adb (Add_Call): Just look at Instantiation_Depth.
	* sem_ch3.adb (Derive_Subprograms): Likewise.
	* sem_warn.adb (Check_References): Remove redundant filtering with
	Instantiation_Depth that follows filtering with
	Instantiation_Location.
	* sinput.adb (Instantiation_Depth): Reuse Instantiation_Location.