[Sema] Add warning when comparing nonnull and null

Currently, we emit warnings in some cases where nonnull function
parameters are compared against null. This patch extends this support
to warn when comparing the result of `returns_nonnull` functions
against null.

More specifically, we will now warn cases like:

int *foo() __attribute__((returns_nonnull));
int main() {
  if (foo() == NULL) {} // warning: will always evaluate to false
}

Differential Revision: http://reviews.llvm.org/D15324

llvm-svn: 255058