Add returns_nonnull to infallible allocators

Postgres memory allocators, by default, ERROR out on memory allocation
failures. An ERROR leads to a longjmp, which means that the caller of
the allocator will never see a NULL return value. We can explicitly let
the compiler know about this behavior by adding the returns_nonnull
attribute to the allocators that follow this behavior. Postgres does
support _extended versions of some of the allocators that take a flaks
argument. The caller can provide the MCXT_ALLOC_NO_OOM flag to these
allocators to request that they return NULL on allocation failure
instead of ERROR-ing out. The _extended allocators cannot be marked as
returns_nonnull because of that.

By using returns_nonnull, we can help the compiler to optimize call
sites.

--
Tristan Partin
PostgreSQL Contributors Team
AWS (https://aws.amazon.com)