atree.ads, atree.adb (Traverse_Func): Walk Field2 last, and eliminate the resulting tail recursion by hand.

2007-12-19  Bob Duff  <duff@adacore.com>

	* atree.ads, atree.adb (Traverse_Func): Walk Field2 last, and eliminate
	the resulting tail recursion by hand. This prevents running out of
	memory on deeply nested concatenations, since Field2 is where the left
	operand of concatenations is stored.
	Fix bug (was returning OK_Orig in some cases). Fix return subtype to
	clarify that it can only return OK or Abandon.

	* sem_res.adb (Resolve_Op_Concat): Replace the recursion on the left
	operand by iteration, in order to avoid running out of memory on
	deeply-nested concatenations. Use the Parent pointer to get back up the
	tree.
	(Resolve_Op_Concat_Arg, Resolve_Op_Concat_First,
	 Resolve_Op_Concat_Rest): New procedures split out of
	Resolve_Op_Concat, so the iterative algorithm in Resolve_Op_Concat is
	clearer.

	* checks.adb (Remove_Checks): Use Traverse_Proc instead of
	Traverse_Func, because the former already takes care of discarding the
	result.

	* errout.adb (First_Node): Use Traverse_Proc instead of Traverse_Func,
	because the former already takes care of discarding the result.
	(Remove_Warning_Messages): Use appropriate subtype for Status and
	Discard

From-SVN: r131070