[Ada] Variable-sized node types -- cleanup

[Ada] Variable-sized node types -- cleanup

gcc/ada/

	* atree.ads, einfo-utils.ads, einfo-utils.adb, fe.h, gen_il.adb,
	gen_il.ads, gen_il-gen-gen_entities.adb,
	gen_il-gen-gen_nodes.adb, sem_ch12.adb, sem_ch3.adb,
	sem_util.adb, sinfo-utils.ads, treepr.adb, types.ads: Clean up
	??? comments and other comments.
	* atree.adb: Clean up ??? comments and other comments.
	(Validate_Node): Fix bug: "Off_0 (N) < Off_L (N)"
	should be "Off_0 (N) <= Off_L (N)".
	* gen_il-gen.adb, gen_il-gen.ads: Clean up ???
	comments and other comments.  Add support for getter-specific
	and setter-specific preconditions.  Detect the error of putting
	a field in the wrong subrange.  Misc cleanup.
	(Node_Field vs. Entity_Field): Clean up Nmake.  Improve
	comments.
	* gen_il-utils.ads: Misc cleanup. Move...
	* gen_il-internals.ads: ... here.
	* gen_il-utils.adb: Misc cleanup. Move...
	* gen_il-internals.adb: ... here.
	* gen_il-fields.ads: Move Was_Default_Init_Box_Association,
	which was in the wrong subrange.  Add comments.  Misc cleanup.
	* gen_il-types.ads: Add Named_Access_Kind.
	* sinfo-cn.adb: Clean up ??? comments and other comments.
	Remove redundant assertions.
	* einfo.ads, sinfo.ads: Clean up ??? comments and other
	comments.  Remove all the comments indicating field offsets.
	These are obsolete now that Gen_IL computes the offsets
	automatically.