[Ada] Simplify conversion from Character to Char_Code

[Ada] Simplify conversion from Character to Char_Code

Replace "Char_Code (Character'Pos (...))" with "Get_Char_Code (...)".
The Get_Char_Code routine is inlined, so there is no performance penalty
when it is called with static actual parameters.

The N_Character_Literal has field Char_Literal_Value of type Unat, but
we should really only store there values from Char_Code type (e.g. there
are no characters with negative ASCII codes). It seems cleaner to use
UI_From_CC and not a more general UI_From_Int when setting the character
literal values.

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

gcc/ada/

	* exp_prag.adb (Expand_Pragma_Import_Or_Interface): Use
	Get_Char_Code.
	* exp_util.adb (Build_Task_Array_Image): Simplify conversion to
	Char_Code.
	(Build_Task_Image_Prefix): Likewise.
	(Build_Task_Record_Image): Likewise.
	* cstand.adb (Create_Standard): Use UI_From_Int instead of
	UI_From_CC.
	* exp_ch11.adb (Expand_N_Exception_Declaration): Likewise.
	* sem_res.adb (Patch_Up_Value): Likewise.
	* stringt.adb (Write_String_Table_Entry): Use Get_Char_Code.