[Ada] Info. gathering in preparation for more efficiency improvements

[Ada] Info. gathering in preparation for more efficiency improvements

gcc/ada/

	* atree.adb: Gather and print statistics about frequency of
	getter and setter calls.
	* atree.ads (Print_Statistics): New procedure for printing
	statistics.
	* debug.adb: Document -gnatd.A switch.
	* gen_il-gen.adb: Generate code for statistics gathering.
	Choose the offset of Homonym early.  Misc cleanup.  Put more
	comments in the generated code.
	* gen_il-internals.ads (Unknown_Offset): New value to indicate
	that the offset has not yet been chosen.
	* gnat1drv.adb: Call Print_Statistics.
	* libgnat/s-imglli.ads: Minor comment fix.
	* output.ads (Write_Int_64): New procedure to write a 64-bit
	value.  Needed for new statistics, and could come in handy
	elsewhere.
	* output.adb (Write_Int_64): Likewise.
	* sinfo.ads: Remove obsolete comment. The xtreeprs program no
	longer exists.
	* types.ads: New 64-bit types needed for new statistics.