ada: Find overlying with slices

ada: Find overlying with slices

A recent change removed rewriting of address of a slice into address of first
component of a slice. Now we need to explicitly detect overlying with slices.
Together, both changes cause warnings to be emitted when slice of a constant
object is overlaid by a variable.

gcc/ada/ChangeLog:

	* sem_util.adb (Find_Overlaid_Entity): Detect overlying with slice.