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.