[Ada] Replace variables with constants in expanded code for task names

[Ada] Replace variables with constants in expanded code for task names

Using constants instead of variables is cleaner both in human-written
and auto-generated code.

Cleanup related to handling of character values in SPARK
counterexamples, which just like the code for names of tasks create
N_Character_Literal nodes.

gcc/ada/

	* exp_util.adb (Build_Task_Array_Image): Declare expanded
	objects as constants.
	(Build_Task_Image_Prefix): Likewise.
	(Build_Task_Record_Image): Likewise.