PR libstdc++/78420 Make std::less etc. yield total order for pointers

[Jonathan Wakely <jwakely dot gcc at gmail dot com>]

In order to meet the total order requirements of [comparisons] p2 we
need to cast unrelated pointers to uintptr_t before comparing them.
Those casts aren't allowed in constant expressions, so only cast
when __builtin_constant_p says the result of the comparison is not a
compile-time constant (because the arguments are not constants, or the
result of the comparison is unspecified). When the result is constant
just compare the pointers directly without casting.

This ensures that the function can be called in constant expressions
with suitable arguments, but still yields a total order even for
otherwise unspecified pointer comparisons.

        PR libstdc++/78420
        * include/bits/stl_function.h (__ptr_rel_ops): New struct providing
        relational operators defining total order on pointers.
        (greater<_Tp*>, less<_Tp*>, greater_equal<_Tp*>, less_equal<_Tp>*):
        New partial specializations for pointers, using __ptr_rel_ops.
        (greater<void>, less<void>, greater_equal<void>, less_equal<void*):
        Dispatch to __ptr_rel_ops when both arguments are pointers.
        * testsuite/20_util/function_objects/comparisons_pointer.cc: New.

Tested powerpc64le-linux. I intend to commit this to trunk and
backport to the active branches, unless anybody sees a problem with
this solution, or has any suggestions for improvement.

Attachment: patch.txt
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