[PATCH 1/2] add -Wuse-after-free

[Martin Sebor]

Patch 1 in the series detects a small subset of uses of pointers
made indeterminate by calls to deallocation functions like free
or C++ operator delete.  To control the conditions the warnings
are issued under the new -Wuse-after-free= option provides three
levels.  At the lowest level the warning triggers only for
unconditional uses of freed pointers and doesn't warn for uses
in equality expressions.  Level 2 warns also for come conditional
uses, and level 3 also for uses in equality expressions.

I debated whether to make level 2 or 3 the default included in
-Wall.  I decided on 3 for two reasons: 1) to raise awareness
of both the problem and GCC's new ability to detect it: using
a pointer after it's been freed, even only in principle, by
a successful call to realloc, is undefined, and 2) because
it's trivial to lower the level either globally, or locally
by suppressing the warning around such misuses.

I've tested the patch on x86_64-linux and by building Glibc
and Binutils/GDB.  It triggers a number of times in each, all
due to comparing invalidated pointers for equality (i.e., level
3).  I have suppressed these in GCC (libiberty) by a #pragma,
and will see how the Glibc folks want to deal with theirs (I
track them in BZ #28521).

The tests contain a number of xfails due to limitations I'm
aware of.  I marked them pr?????? until the patch is approved.
I will open bugs for them before committing if I don't resolve
them in a followup.

Martin
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