Building llvm with c++23 seems to not yet be supported.
I'm keen on building it anyways to use 'import std;' in NebulaStream which uses LLVM with MLIR as a dependency.

I'm making patches to get the build working based on the llvmorg-20.1.8 release, and many of them seem to still be relevant when building against the release candidates for 21 and 22.

When switching the LLVM_REQUIRED_CXX_STANDARD variable to 23 on the 20.1.8 release many errors are encountered.
Most of the issues relate to the constexpr changes of unique_ptr in c++23.

I'd like to make a PR to share the changes I am making to get this working and see if any of them could be useful for the next 20.x.x release or other major releases.