Reland Produce warning for performing pointer arithmetic on a null pointer.

Summary:
Test and produce warning for subtracting a pointer from null or subtracting
null from a pointer.

This reland adds the functionality that the warning is no longer reusing an
existing warning, it has different wording for C vs C++ to refect the fact
that nullptr-nullptr has defined behaviour in C++,  it is suppressed
when the warning is triggered by a system header and adds
-Wnull-pointer-subtraction to allow the warning to be controlled.  -Wextra
implies -Wnull-pointer-subtraction.

Author: Jamie Schmeiser <schmeise@ca.ibm.com>
Reviewed By: efriedma (Eli Friedman), nickdesaulniers (Nick Desaulniers)
Differential Revision: https://reviews.llvm.org/D98798