## Purpose
This patch is one in a series of code-mods that annotate LLVM’s public
interface for export. This patch annotates the `llvm-c` interface with a
new `LLVM_C_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 `llvm-c/Visibility.h` header file that defines a new
`LLVM_C_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_LLVM_C_EXPORT_ANNOTATIONS` `#cmakedefine` that
is used to gate the definition of `LLVM_C_ABI`.
3. Remove the existing `LLVM_C_ABI` definition from
`llvm/Support/Compiler.h`. Update the small number of `LLVM_C_ABI`
references to get it from the new `llvm-c/Visibility.h` header.
4. Code-mod annotate the public `llvm-c` interface using the [Interface
Definition Scanner (IDS)](https://github.com/compnerd/ids) tool.
5. Format the changes with `clang-format`.
## Background
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
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