[PATCH] improve validation of attribute arguments (PR c/78666)

[Martin Sebor]

GCC has gotten better at detecting conflicts between various
attributes but it still doesn't do a perfect job of detecting
similar problems due to mismatches between contradictory
arguments to the same attribute.  For example,

   __attribute ((alloc_size (1))) void* allocate (size_t, size_t);

followed by

   __attribute ((alloc_size (2))) void* allocate (size_t, size_t);

is accepted with the former overriding the latter in calls to
the function.  Similar problem exists with a few other attributes
that take arguments.

The attached change adds a new utility function that checks for
such mismatches and issues warnings.  It also adds calls to it
to detect the problem in attributes alloc_align, alloc_size, and
section.  This isn't meant to be a comprehensive fix but rather
a starting point for one.

Tested on x86_64-linux.

Martin

PS I ran into this again while debugging some unrelated changes
and wondering about the behavior in similar situations to mine.
Since the behavior seemed clearly suboptimal I figured I might
as well fix it.

PPS The improved checking triggers warnings in a few calls to
__builtin_has_attribute due to apparent conflicts.  I've xfailed
those in the test since it's a known issue with some existing
attributes that should be fixed at some point.  Valid uses of
the built-in shouldn't trigger diagnostics except for completely
nonsensical arguments.  Unfortunately, the line between valid
and completely nonsensical is a blurry one (GCC either issues
errors, or -Wattributes, or silently ignores some cases
altogether, such as those that are the subject of this patch)
and there is no internal mechanism to control the response.
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