ada: Make Exp_Ch9.Build_Task_Allocate_Block a function

ada: Make Exp_Ch9.Build_Task_Allocate_Block a function

This streamlines the code in the callers.  No functional changes.

gcc/ada/ChangeLog:

	* exp_ch9.ads (Build_Task_Allocate_Block): Change to function and
	remove first formal parameter.
	* exp_ch9.adb (Build_Task_Allocate_Block): Likewise.  Return an
	anonymous list of nodes.
	* exp_aggr.adb (Convert_Aggr_In_Allocator): Adjust to above change.
	* exp_ch4.adb (Expand_N_Allocator): Likewise.
	* exp_ch6.adb (Make_Build_In_Place_Call_In_Allocator): Likewise.