ada: Streamline implementation of masters in Exp_Ch9

ada: Streamline implementation of masters in Exp_Ch9

The incidental discovery of an old issue and its resolution has exposed the
convoluted handling of masters in Exp_Ch9, which uses two totally different
approaches to achieve the same goal, respectively in Build_Master_Entity and
Build_Class_Wide_Master, the latter being quite hard to follow. The handling
of activation chains for extended return statements is also a bit complex.

This gets rid of the second approach entirely for masters, as well as makes
the handling of activation chains uniform for all nodes.

No functional changes.

gcc/ada/ChangeLog:

	* gen_il-gen-gen_nodes.adb (N_Extended_Return_Statement): Add
	Activation_Chain_Entity semantic field.
	* exp_ch3.adb (Build_Master): Use Build_Master_{Entity,Renaming} in
	all cases.
	(Expand_N_Object_Declaration): Small tweak.
	* exp_ch6.adb (Make_Build_In_Place_Iface_Call_In_Allocator): Use
	Build_Master_{Entity,Renaming} to build the master.
	* exp_ch7.adb (Expand_N_Package_Declaration): Do not guard the call
	to Build_Task_Activation_Call for the sake of consistency.
	* exp_ch9.ads (Build_Class_Wide_Master): Delete.
	(Find_Master_Scope): Likewise.
	(Build_Protected_Subprogram_Call_Cleanup): Move to...
	(First_Protected_Operation): Move to...
	(Mark_Construct_As_Task_Master): New procedure.
	* exp_ch9.adb (Build_Protected_Subprogram_Call_Cleanup): ...here.
	(First_Protected_Operation): ...here.
	(Build_Activation_Chain_Entity): Streamline handling of extended
	return statements.
	(Build_Class_Wide_Master): Delete.
	(Build_Master_Entity): Streamline handling of extended return
	statements and call Mark_Construct_As_Task_Master on the context.
	(Build_Task_Activation_Call): Assert that the owner is not an
	extended return statement.
	(Find_Master_Scope): Delete.
	(Mark_Construct_As_Task_Master): New procedure.
	* sem_ch3.adb (Access_Definition): Use Build_Master_{Entity,Renaming}
	in all cases to build a master.
	* sem_ch6.adb (Check_Anonymous_Return): Rename to...
	(Check_Anonymous_Access_Return_With_Tasks): ...this.  At the end,
	call Mark_Construct_As_Task_Master on the parent node.
	(Analyze_Subprogram_Body_Helper): Adjust to above renaming.
	(Create_Extra_Formals): Do not set Has_Master_Entity here.
	* sinfo.ads (Activation_Chain_Entity): Adjust description.