ada: Simplify construction of internal string from characters

Reuse a variant of Store_String_Char routine that accepts a Character parameter
and internally calls Get_Char_Code. Code cleanup.

gcc/ada/ChangeLog:

	* exp_attr.adb (Expand_N_Attribute_Reference): Store chars without
	converting the actual parameters.
	* exp_ch11.adb (Null_String): Likewise.
	* exp_disp.adb (Make_DT): Likewise.
	* exp_util.adb (Fully_Qualified_Name_String): Likewise.
	* sem_attr.adb (Analyze_Attribute): Likewise.
	* sem_dist.adb (Full_Qualified_Name): Likewise.
	* sem_prag.adb (Process_Interface_Name, Set_Encoded_Interface_Name):
	Likewise.
	* stringt.adb (Store_String_Chars): Likewise.