There are three kinds of units that require code generation: the main
unit, its corresponding spec and generic instances needed by the main
unit.

Previously the main unit and its corresponding spec were flagged as
requiring code generation just before calling the backend, while
instance units were flagged while they were created, which was
inconsistent.

Now all of them are flagged as soon as they are created, which both
appears to be simpler and makes the Generate_Code flag valid all the
time.

Tested on x86_64-pc-linux-gnu, committed on trunk

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.