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

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