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)