[Ada] Build static dispatch tables always at the end of declarative part

[Ada] Build static dispatch tables always at the end of declarative part

The static dispatch tables of library-level tagged types are either built
on the first object declaration or at the end of the declarative part of
the package spec or body. There is no real need for the former case, and
the tables are not built for other constructs that freeze (tagged) types.

Therefore this change removes the former case, thus causing the tables to
be always built at the end of the declarative part; that's orthogonal to
freezing and the tagged types are still frozen at the appropriate place.

Moreover, it wraps the code in the Actions list of a freeze node (like
for the nonstatic case) so that it is considered elaboration code by the
processing done in Sem_Elab and does not disturb it.

No functional changes.

gcc/ada/

	* exp_ch3.adb (Expand_Freeze_Record_Type): Adjust comment.
	(Expand_N_Object_Declaration): Do not build static dispatch tables.
	* exp_disp.adb (Make_And_Insert_Dispatch_Table): New procedure.
	(Build_Static_Dispatch_Tables): Call it to build the dispatch tables
	and wrap them in the Actions list of a freeze node.