[Bug c/16351] NULL dereference warnings

[law at redhat dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=16351

--- Comment #44 from Jeffrey A. Law <law at redhat dot com> ---
Duh, I should have remembered that we had returns_nonnull because I wrote a
dataflow pass to find any NULLs that flow to such statements :-)

The pieces that are missing are tagging functions with that attribute when we
can prove they never return NULL.  We'll be relying a lot of IPA to present
functions in the right order, but it'd be a good improvement.