[Sema] Revert the change in r200622 that allowed integer casts to silence...

[Sema] Revert the change in r200622 that allowed integer casts to silence -Wnon-literal-null-conversion in C code.

It is actually useful to warn in such cases, thanks to Dmitri for pushing on this and making us see the light!

Related to rdar://15925483 and rdar://15922612. The latter radar is where the usefulness of the warning is most clear.

llvm-svn: 201165