ada: Fix internal error with Atomic Volatile_Full_Access object

The initial implementation of the GNAT aspect/pragma Volatile_Full_Access
made it incompatible with Atomic, because it was not decided whether the
read-modify-write sequences generated by Volatile_Full_Access would need
to be implemented atomically when Atomic was also specified, which would
have required a compare-and-swap primitive from the target architecture.

But Ada 2022 introduced Full_Access_Only and retrofitted it into Atomic
in the process, answering the above question by the negative, so the
incompatibility between Volatile_Full_Access and Atomic was lifted in
Ada 2012 as well, unfortunately without adjusting the implementation.

gcc/ada/

	* gcc-interface/trans.cc (get_atomic_access): Deal specifically with
	nodes that are both Atomic and Volatile_Full_Access in Ada 2012.