Add support for __attribute__((returns_nonnull))

Here is a patch which adds support for the returns_nonnull attribute
alongside all the other attributes we optionally support.

I recently wound up in a situation where I was checking for NULL return
values of a function that couldn't ever return NULL because the
inability to allocate memory was always elog(ERROR)ed (aborted).

I didn't go through and mark anything, but I feel like it could be
useful for people going forward, including myself.

--
Tristan Partin
Neon (https://neon.tech)