ada: Fix generic instantiation of sibling package

ada: Fix generic instantiation of sibling package

The compiler would crash because it is failing at setting up the scope
stack correctly for a generic instantiation of a sibling package within
a child package instance. In this case, the parent instance isn't
explicitly referenced and it must be found differently.

gcc/ada/

	* sem_ch12.adb (Instantiate_Package_Body): Correctly find the
	parent instance to place on the scope stack.