Add malloc attribute to memory allocation functions

Given that we now have a tree that compiles fine against 
-Werror=mismatched-dealloc, we need to make sure that we don't regress. 
By adding the malloc attribute[0], we can protect against regressions, 
enable more accurate code coverage with -fanalyzer, and allow the 
compiler to do some optimizations.

Let me know if I missed any allocators. Additionally, does it make sense 
to add the attribute to the various memory context allocator functions 
like AllocSetAlloc(), or only on the higher level functions.

[0]: https://gcc.gnu.org/onlinedocs/gcc/Common-Attributes.html#index-malloc

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