Introduce exception handler ABI #1 to ensure single release, no access
after release of reraised Machine_Occurrences, and no failure to
re-reraise a Machine_Occurrence.

Unlike Ada exceptions, foreign exceptions do not get a new
Machine_Occurrence upon reraise, but each handler would delete the
exception upon completion, normal or exceptional, save for the case of
a 'raise;' statement within the handler, that avoided the delete by
clearing the exception pointer that the cleanup would use to release
it.  The cleared exception pointer might then be used by a subsequent
reraise within the same handler.  Get_Current_Excep.all would also
expose the Machine_Occurrence to reuse by Reraise_Occurrence, even for
native exceptions.

Under ABI #1, Begin_Handler_v1 claims responsibility for releasing an
exception by saving its cleanup and setting it to Claimed_Cleanup.
End_Handler_v1 restores the cleanup and runs it, as long as it isn't
still Claimed_Cleanup (which indicates an enclosing handler has
already claimed responsibility for releasing it), and as long as the
same exception is not being propagated up (the next handler of the
propagating exception will then claim responsibility for releasing
it), so reraise no longer needs to clear the exception pointer, and it
can just propagate the exception, just like Reraise_Occurrence.

ABI #1 is fully interoperable with ABI #0, i.e., exception handlers
that call the #0 primitives can be linked together with ones that call
the #1 primitives, and they will not misbehave.  When a #1 handler
claims responsibility for releasing an exception, even #0 reraises
dynamically nested within it will refrain from releasing it.  However,
when a #0 handler is a handler of a foreign exception that would have
been responsible for releasing it with #1, a Reraise_Occurrence of
that foreign or other Machine_Occurrence-carrying exception may still
cause the exception to be released multiple times, and to be used
after it is first released, even if other handlers of the foreign
exception use #1.


for  gcc/ada/ChangeLog

	* libgnat/a-exexpr.adb (Begin_Handler_v1, End_Handler_v1): New.
	(Claimed_Cleanup): New.
	(Begin_Handler, End_Handler): Document.
	* gcc-interface/trans.c (gigi): Switch to exception handler
	ABI #1.
	(Exception_Handler_to_gnu_gcc): Save the original cleanup
	returned by begin handler, pass it to end handler, and use
	EH_ELSE_EXPR to pass a propagating exception to end handler.
	(gnat_to_gnu): Leave the exception pointer alone for reraise.
	(add_cleanup): Handle EH_ELSE_EXPR, require it by itself.

From-SVN: r274029