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.

llvm-svn: 212874
diff --git a/clang/test/CodeGen/nonnull.c b/clang/test/CodeGen/nonnull.c
index b39a14c..4d6cc45 100644
--- a/clang/test/CodeGen/nonnull.c
+++ b/clang/test/CodeGen/nonnull.c
@@ -15,3 +15,9 @@
   *x = 0;
 }
 
+static int a;
+// CHECK: define nonnull i32* @bar3()
+int * bar3() __attribute__((returns_nonnull))  {
+  return &a;
+}
+