[Ada] Avoid creating a finalization wrapper block for functions

This patch fixes a bug whereby if a function body has local objects that
are finalizable, and has a return statement that requires generation of
transient finalizable temps, and there are dynamic-sized objects
requiring pushing/popping the (primary) stack at run time, and the
function body has exception handlers, then incorrect code is
generated. In particular, the transient objects are finalized after they
have been deallocated. This can cause seg faults, corrupted heap, and
the like.

Note that if there are no dynamic-sized objects, then the bug does
not occur, because the back end allocates objects of compile-time-known
size based on where they are referenced, rather than which local
block they are declared in.

This patch relies on the fact that an At_End handler and regular
exception handlers CAN coexist in the same handled statement sequence,
which was not true some years ago.

gcc/ada/

	* exp_ch7.adb (Wrap_HSS_In_Block): Do not create a new block in
	the case of function bodies. We include all subprogram bodies,
	because it's harmless for procedures. We cannot easily avoid
	creating this block in ALL cases, because some transformations
	of (e.g.) task bodies end up moving some code such that the
	wrong exception handlers apply to that code.
	(Build_Finalizer_Call): Remove code for creating a new block.
	This was unreachable code, given that Wrap_HSS_In_Block has
	already done that, but with the above change to
	Wrap_HSS_In_Block, this code becomes reachable, and triggers
	essentially the same bug.
	* exp_ch7.ads: Adjust comment.

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