[Ada] Simplify handling of Generate_Code flag for compilation units

[Ada] Simplify handling of Generate_Code flag for compilation units

gcc/ada/

	* gnat1drv.adb (Gnat1drv): Remove flagging of main unit and its
	corresponding spec as requiring code generation; now the flags
	are set much earlier.
	* lib-load.adb (Load_Main_Source): Set Generate_Code flag on the
	main unit source.
	(Make_Instance_Unit): Copy Generate_Code flag from the main unit
	to instance units.
	* lib-writ.adb (Write_ALI): Remove redundant condition;
	Generate_Code flag is always set for the main unit.
	* par-load.adb (Load): Set Generate_Code flag on the main unit's
	corresponding spec, if any.