[Ada] Fix spurious error for aggregate with box component choice

[Ada] Fix spurious error for aggregate with box component choice

It comes from the Volatile_Full_Access (or Atomic) aspect: the aggregate is
effectively analyzed/resolved twice and this does not work.  It is fixed by
calling Is_Full_Access_Aggregate before resolution.

gcc/ada/

	* exp_aggr.adb (Expand_Record_Aggregate): Do not call
	Is_Full_Access_Aggregate here.
	* freeze.ads (Is_Full_Access_Aggregate): Delete.
	* freeze.adb (Is_Full_Access_Aggregate): Move to...
	(Freeze_Entity): Do not call Is_Full_Access_Aggregate here.
	* sem_aggr.adb (Is_Full_Access_Aggregate): ...here
	(Resolve_Aggregate): Call Is_Full_Access_Aggregate here.