PR10837: Warn if a null pointer constant is formed by a zero integer constant
expression that is not a zero literal, in C. This is a different, and more
targeted, approach than that in r194540.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@195303 91177308-0d34-0410-b5e6-96231b3b80d8