[PATCH] issue consistent warning for past-the-end array stores (PR 91457)

[Martin Sebor]

With the recent enhancement to the strlen handling of multibyte
stores the g++.dg/warn/Warray-bounds-4.C for zero-length arrays
started failing on hppa (and probably elsewhere as well).  This
is partly the result of the added detection of past-the-end
writes into the strlen pass which detects more instances of
the problem than -Warray-bounds.  Since the IL each warning
works with varies between targets, the same invalid code can
be diagnosed by one warning one target and different warning
on another.

The attached patch does three things:

1) It enhances compute_objsize to also determine the size of
a flexible array member (and its various variants), including
from its initializer if necessary.  (This resolves 91457 but
introduces another warning where was previously just one.)
2) It guards the new instance of -Wstringop-overflow with
the no-warning bit on the assignment to avoid warning on code
that's already been diagnosed.
3) It arranges for -Warray-bounds to set the no-warning bit on
the enclosing expression to keep -Wstringop-overflow from issuing
another warning for the same problem.

Testing the compute_objsize enhancement to bring it up to par
with -Warray-bounds in turn exposed a weakness in the latter
warning for flexible array members.  Rather than snowballing
additional improvements into this one I decided to put that
off until later, so the new -Warray-bounds test has a bunch
of XFAILs.  I'll see if I can find the time to deal with those
either still in stage 1 or in stage 3 (one of them is actually
an ancient regression).

Martin

PS I imagine the new get_flexarray_size function will probably
need to move somewhere more appropriate so that other warnings
(like -Warray-bounds to remove the XFAILs) and optimizations
can make use of it.
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