Add nonnull in CodeGen for __attribute__((returns_nonnull))

As a follow-up to r212835, also add the LLVM nonnull function attribute when
__attribute__((returns_nonnull)) is provided.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@212874 91177308-0d34-0410-b5e6-96231b3b80d8