Polymorphic allocators in LLVM 16

The C++17 memory resources functions have been merged to what might eventually become LLVM 16:
rG243da90ea535 (llvm.org)

That might mean that Octave might be able to use std::pmr::polymorphic_allocator while linking to libc++ version 16 (once it will be released probably in spring of 2023). Up until now only libstdc++ did implement those functions in the FOSS world.