[libcxxabi] [llvm] [llvm] annotate interfaces in Demangle for DLL export (PR #147564)

[Andrew Rogers via llvm-commits]

andrurogerz wrote:

#149163 cleans-up `DemangleConfig.h` to use definitions from `Compiler.h.` I think this solution will be more maintainable. @nikic, @compnerd if you two OK with that PR, then I will rebase this change on top of it and alias `DEMANGLE_ABI` to `LLVM_ABI` in `DemangleConfig.h`. This will eliminate the need for the new `Demangle/Visbility.h` header and keep the status-quo with the `cp-to-llvm.sh` script.

https://github.com/llvm/llvm-project/pull/147564