ada: Fix latent issue in support for protected entries

ada: Fix latent issue in support for protected entries

The problem is that, unlike for protected subprograms, the expansion of
cleanups for protected entries is not delayed when they contain package
instances with a body, so the cleanups are generated twice and this may
yield two finalizers if the secondary stack is used in the entry body.

This restores the delaying, which uncovers the missing propagation of the
Uses_Sec_Stack flag as is done for protected subprograms, which in turn
requires using a Corresponding_Spec field as for protected subprograms.

This also gets rid of the Delay_Subprogram_Descriptors flag on entities,
whose only remaining use in Expand_Cleanup_Actions was unreachable.

The last change is to unconditionally reset the scopes in the case of
protected subprograms when they are expanded, as is done in the case of
protected entries.  This makes it possible to remove the code adjusting
the scope on the fly in Cleanup_Scopes but requires a few adjustments.

gcc/ada/

	* einfo.ads (Delay_Subprogram_Descriptors): Delete.
	* gen_il-fields.ads (Opt_Field_Enum): Remove
	Delay_Subprogram_Descriptors.
	* gen_il-gen-gen_entities.adb (Gen_Entities): Likewise.
	* gen_il-gen-gen_nodes.adb (N_Entry_Body): Add Corresponding_Spec.
	* sinfo.ads (Corresponding_Spec): Document new use.
	(N_Entry_Body): Likewise.
	* exp_ch6.adb (Expand_Protected_Object_Reference): Be prepared for
	protected subprograms that have been expanded.
	* exp_ch7.adb (Expand_Cleanup_Actions): Remove unreachable code.
	* exp_ch9.adb (Build_Protected_Entry): Add a local variable for the
	new block and propagate Uses_Sec_Stack from the corresponding spec.
	(Expand_N_Protected_Body) <N_Subprogram_Body>: Unconditionally reset
	the scopes of top-level entities in the new body.
	* inline.adb (Cleanup_Scopes): Do not adjust the scope on the fly.
	* sem_ch9.adb (Analyze_Entry_Body): Set Corresponding_Spec.
	* sem_ch12.adb (Analyze_Package_Instantiation): Remove obsolete code
	setting Delay_Subprogram_Descriptors and tidy up.
	* sem_util.adb (Scope_Within): Deal with protected subprograms that
	have been expanded.
	(Scope_Within_Or_Same): Likewise.