[PATCH] correct handling of variable offset minus constant in -Warray-bounds (PR 100137)

[Martin Sebor]

-Warray-bounds relies on similar logic as -Wstringop-overflow et al.,
but using its own algorithm, including its own bugs such as PR 100137.
The attached patch takes the first step toward unifying the logic
between the warnings.  It changes a subset of -Warray-bounds to call
compute_objsize() to detect out-of-bounds indices.  Besides fixing
the bug this also nicely simplifies the code and improves
the consistency between the informational messages printed by both
classes of warnings.

The changes to the test suite are extensive mainly because of
the different format of the diagnostics resulting from slightly
tighter bounds of offsets computed by the new algorithm, and in
smaller part because the change lets -Warray-bounds diagnose some
problems it previously missed due to the limitations of its own
solution.

The false positive reported in PR 100137 is a 10/11/12 regression
but this change is too intrusive to backport.  I have a smaller
and more targeted patch I plan to backport in its stead.

Tested on x86_64-linux.

Martin
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