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

[Andrew Rogers via llvm-commits]

andrurogerz wrote:

Thanks @nikic and @ldionne for having a look at this PR. I don't think I have a definitive answer on whether or not Demangle can safely take a dependency on `llvm/Support/Compiler.h` for the `LLVM_ABI` definition. Digging a bit more, though, I do see [this comment](https://github.com/llvm/llvm-project/blob/main/llvm/include/llvm/Demangle/DemangleConfig.h#L9-L11) in `llvm/Demangle/DemangleConfig.h`:
```
// This file contains a variety of feature test macros copied from
// include/llvm/Support/Compiler.h so that LLVMDemangle does not need to take
// a dependency on LLVMSupport.
```
So it seems pretty clear that I should remove the top commit from this stack and go back to independently defined `DEMANGLE_ABI` `llvm/Demangle/Visibility.h`. Additionally, I am considering striking this new file entirely in favor of just putting the definition in `DemangleConfig.h`. This way we don't pollute `libcxxabi` with additional files and don't add to the complexity of the copy script. @compnerd does this approach sound OK to you?

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