Make -Wnull-conversion more useful.

When a null constant is used in a macro, walk through the macro stack to
determine where the null constant is written and where the context is located.
Only warn if both locations are within the same macro expansion.  This helps
function-like macros which involve pointers be treated as if they were
functions.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@260776 91177308-0d34-0410-b5e6-96231b3b80d8