[PATCH] avoid false positives due to compute_objsize (PR 95353)

[Martin Sebor]

The compute_objsize() function started out as a thin wrapper around
compute_builtin_object_size(), but over time developed its own
features to compensate for the other function's limitations (such
as its inability to work with ranges).  The interaction of these
features and the limitations has started to become increasingly
problematic as the former function is used in more contexts.

A complete "fix" for all the problems (as well as some other
limitations) that I'm working on will be more extensive and won't
be appropriate for backports.  Until then, the attached patch
cleans up the extensions compute_objsize() has accumulated over
the years to avoid a class of false positives.

To make the warnings issued based on the results of the function
easier to understand and fix, the patch also adds an informative
message to many instances of -Wstringop-overflow to point to
the object to which the warning refers.  This is especially
helpful when the object is referenced by a series of pointer
operations.

Tested by boostrapping on x86_64-linux and building Binutils/GDB,
Glibc, and the Linux kernel with no new warnings.

Besides applying it to trunk I'm looking to backport the fix to
GCC 10.

Martin
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