[Ada] Overriding errors on renamings and instances overriding predefined operators

[Ada] Overriding errors on renamings and instances overriding predefined operators

gcc/ada/

	* sem_ch6.ads (Can_Override_Operator): Function declaration
	moved from package body to package spec.
	* sem_ch6.adb (Check_Overriding_Indicator): Now use test of
	whether the subprogram's Chars is an operator name, to handle
	cases of function instances whose entity is
	N_Defining_Identifier rather than N_Defining_Operator_Symbol.
	(Can_Override_Operator): Function declaration moved to package
	spec.  Now use test of whether the subprogram's Chars is an
	operator name, to handle cases of function instances whose
	entity is N_Defining_Identifier rather than
	N_Defining_Operator_Symbol.
	* sem_ch8.adb (Analyze_Renamed_Subprogram): Check for
	possibility of an overridden predefined operator, and suppress
	the "not overriding" message in that case.