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

[Louis Dionne via llvm-commits]

https://github.com/ldionne approved this pull request.

This patch looks correct to me (well, the small libc++abi part -- I didn't carefully check the rest).

We don't want to depend on LLVM's support library from libc++abi, so IMO defining a standalone `DEMANGLE_ABI` macro and noop-ing it from `libcxxabi/src/demangle/Visibility.h` looks like the right approach to me.

I will mention that I'm getting more and more worried about the increase in complexity around this duplicated code, though. What started as a simple "synchronize by copying" script now needs to make sure to copy some headers but not others (which it does implicitly). This is not specifically against this patch, but I think we should invest in refactoring this such that we have a single copy of this code.

The approval is for libc++abi bits -- like I said I didn't carefully look at the code under `llvm/include`.

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