ada: Tune recent change for warning about unsupported overlays

ada: Tune recent change for warning about unsupported overlays

Fix crash occurring when overlay applies to protected component and expansion
is disabled, e.g. because of semantic checking mode (switch -gnatc) or because
the compiler is running in GNATprove mode.

Also, simply pick the type of overlaid object from the attribute prefix itself.

gcc/ada/ChangeLog:

	* sem_util.adb (Find_Overlaid_Entity): Don't call Etype on empty Ent;
	tune style; move computation of Overl_Typ out of the loop.