The attached patch fixes the outstanding cases mentioned in comment
10 on bug c++/67376.  While testing the fix I uncovered a number of
other related problems without which the test would have been
incomplete.  They include:

PR c++/70170 - [6 regression] bogus not a constant expression error
     comparing pointer to array to null
PR c++/70172 - incorrect reinterpret_cast from integer to pointer
     error on invalid constexpr initialization
PR c++/60760 - arithmetic on null pointers should not be allowed
     in constant expressions

In addition, I include a fix for the issue below that I also came
across while testing the patch and that makes root causing constexpr
problems due to out-of-bounds array subscripts easier:
PR c++/70228 - insufficient detail in diagnostics for a constexpr
     out of bounds array subscript

In a discussion of bug 70170 between those CC'd Marek posted
a prototype patch for match.pd.  While the patch seems to do
the right thing as far as the bug goes, like my own first attempt
at a fix in const-fold.c it caused a couple of regressions (in
pr21294.c and in pr44555.c).  Since I'm not yet familiar enough
with match.pd, in the interest of time I solved those regressions
in const-fold.c rather than in match.pd.

Tested on x86_64.

Martin