MemoryBuiltins: start using properties of functions

Prior to this change, we relied on the hard-coded list for all of the
information performed by MemoryBuiltins. With this change, we're able to
start relying on properites of functions described in attributes, which
opens the door to out-of-tree compilers being able to describe their
allocator functions to LLVM's optimizer logic without having to register
their implementation details with LLVM.

Differential Revision: https://reviews.llvm.org/D123090