[Sema] For -Wnon-literal-null-conversion warning, look through integer casts, which are used
by some projects in their null macro.

rdar://15925483

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@200521 91177308-0d34-0410-b5e6-96231b3b80d8