[Ada] Propagate null-exclusion to anonymous access types

When analyzing an array or record type declaration whose component has a
constrained access type, e.g.:

   type Buffer_Acc is not null access all String;

   type Buffer_Rec is record
      Data : Buffer_Acc (1 .. 10);
   end record;

   type Buffer_Arr is array (Boolean) of Buffer_Acc (1 .. 10);

we propagated various properties of the unconstrained access type (e.g.
the designated type, access-to-constant flag), but forgot to propagate
the null-exclusion.

For GNAT it didn't make a big difference, because the (anonymous)
component type was never subject to legality checks. The "value
tracking" optimisation machinery, which also deals with null values,
only works for entire objects and doesn't care about components.
However, GNATprove uses this flag when an access-to-component object is
dereferenced.

gcc/ada/

	* sem_ch3.adb (Constrain_Access): Propagate null-exclusion flag
	from parent type.

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