## Purpose
This patch is one in a series of code-mods that annotate LLVM’s public
interface for export. This patch annotates the `Demangle` interface with
a new `DEMANGLE_ABI` annotation, which behaves like the `LLVM_ABI`. This
annotation currently has no meaningful impact on the LLVM build;
however, it is a prerequisite to support an LLVM Windows DLL (shared
library) build.
## Overview
1. Add a new `Demangle/Visibility.h` header file that defines a new
`DEMANGLE_ABI` macro. The macro resolves to the proper DLL export/import
annotation on Windows and a "default" visibility annotation elsewhere.
2. Add a new `LLVM_ENABLE_DEMANGLE_EXPORT_ANNOTATIONS ` `#cmakedefine`
that is used to gate the definition of `DEMANGLE_ABI`.
3. Code-mod annotate the public `Demangle` interface using the
[Interface Definition Scanner (IDS)](https://github.com/compnerd/ids)
tool.
4. Manually fix-up `#include` statements for consistency.
5. Format the changes with `clang-format`.
6. Add a "stub" version of `Visibility.h` under `libcxxabi/src/demangle`
that always defines `DEMANGLE_ABI` to nothing.
7. Update the canonical `libcxxabi/src/demangle/ItaniumDemangle.h`
instead of the llvm copy, and run `cp-to-llvm.sh` to ensure the llvm
copy matches. NOTE: we rely on `ccp-to-llvm.sh` not copying
`Visibillity.h` as is already the case for `DemangleConfig.h`.
## Background
This PR follows the pattern established with the `llvm-c` changes made
in #141701.
This effort is tracked in #109483. Additional context is provided in
[this
discourse](https://discourse.llvm.org/t/psa-annotating-llvm-public-interface/85307),
and documentation for `LLVM_ABI` and related annotations is found in the
LLVM repo
[here](https://github.com/llvm/llvm-project/blob/main/llvm/docs/InterfaceExportAnnotations.rst).
## Validation
Local builds and tests to validate cross-platform compatibility. This
included llvm, clang, and lldb on the following configurations:
- Windows with MSVC
- Windows with Clang
- Linux with GCC
- Linux with Clang
- Darwin with Clang
---------
Co-authored-by: Louis Dionne <ldionne.2@gmail.com>
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