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.  Reuse existing warning that this is undefined
behaviour.  Also add unit test for both warnings.

Reformat to satisfy clang-format.

Respond to review comments:  add additional test.

Respond to review comments:  Do not issue warning for nullptr - nullptr
in C++.

Fix indenting to satisfy clang-format.

Respond to review comments:  Add C++ tests.

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