[FunctionAttrs] try to extend nonnull-ness of arguments from a callsite back to its parent function

As discussed here:
http://lists.llvm.org/pipermail/llvm-dev/2016-December/108182.html
...we should be able to propagate 'nonnull' info from a callsite back to its parent.

The original motivation for this patch is our botched optimization of "dyn_cast" (PR28430),
but this won't solve that problem.

The transform is currently disabled by default while we wait for clang to work-around
potential security problems:
http://lists.llvm.org/pipermail/cfe-dev/2017-January/052066.html

Differential Revision: https://reviews.llvm.org/D27855

llvm-svn: 294998