ada: Fix incorrect handling of packed array with aliased composite components

ada: Fix incorrect handling of packed array with aliased composite components

The problem is that the handling of the interaction between packing and
aliased/atomic/independent components of an array type is tied to that of
the interaction between a component clause and aliased/atomic/independent
components, although the semantics are different: packing is a best effort
thing, whereas a component clause must be honored or else an error be given.

This decouples the two handlings, but retrofits the separate processing of
independent components done in both cases into the common code and changes
the error message from "minimum allowed is" to "minimum allowed value is"
for the sake of consistency with the aliased/atomic processing.

gcc/ada/

	* freeze.adb (Freeze_Array_Type): Decouple the handling of the
	interaction between packing and aliased/atomic components from
	that of the interaction between a component clause and aliased/
	atomic components, and retrofit the processing of the interaction
	between the two characteristics and independent components into
	the common processing.

gcc/testsuite/ChangeLog:

	* gnat.dg/atomic10.adb: Adjust.