[Ada] Fix attributes Version and Body_Version for concurrent units

[Pierre-Marie de Rodat]

Attributes Version and Body_Version can be prefixed by program units,
which include protected units and task units. Those attributes were
wrongly accepted when prefixed by protected subtype, task subtype and by
any object of a concurrent type.

Tested on x86_64-pc-linux-gnu, committed on trunk

gcc/ada/

	* sem_attr.adb (Check_Program_Unit): Fix references to
	Concurrent_Kind and Is_Concurrent_Type; avoid repeated calls to
	Entity.
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