[Ada] Restore full generation of static dispatch tables with -gnatzr

[Ada] Restore full generation of static dispatch tables with -gnatzr

The -gnatzr switch triggers the creation of distribution stubs for use
by the implementation of PolyORB.  Now these stubs declare tagged types
and are generated at the very end of the analysis of compilation units,
after the static dispatch tables have been built, so these tables are
missing for the tagged types of the stubs.

Therefore this change defers the generation of static dispatch tables
for compilation units, which is the common case, until after the stubs
are (potentially) generated.  For the other cases, in particular the
generic instances that are not compilation units, nothing is changed.

gcc/ada/

	* exp_ch7.adb (Expand_N_Package_Body): Build static dispatch
	tables only for units that are not compilation units, unless
	they are generic instances.  Do not push a scope for this.
	(Expand_N_Package_Declaration): Build static dispatch tables
	only for units that are both not compilation units and generic
	instances.
	* exp_disp.adb (Build_Static_Dispatch_Tables): Remove redundant
	early return.  Push a scope for package bodies.
	* sem_ch10.adb: Add with and use clauses for Exp_Disp.
	(Analyze_Compilation_Unit): Build static dispatch tables here.