[llvm] annotate interfaces in llvm-c for DLL export

[andrurogerz]

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. 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.
3. Code-mod annotate the public llvm-c interface using the Interface Definition Scanner (IDS) tool.
4. Format the changes with clang-format.

Background

This effort is tracked in #109483. Additional context is provided in this discourse, and documentation for LLVM_ABI and related annotations is found in the LLVM repo here.

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

[compnerd]

I really would like to see this be simplified for LLVM-C, perhaps something like:

#if defined(LLVM_C_STATIC)
# define LLVM_C_ABI
#else
# if defined(_WIN32) && !defined(__MINGW32__)
#   if defined(LLVM_C_EXPORTS)
#     define LLVM_C_ABI __declspec(dllexport)
#   else
#     define LLVM_C_ABI __declspec(dllimport)
#   endif
# else
#   if defined(__has_attribute) && __has_attribute(visibility)
#     define LLVM_C_ABI __attribute__((__visibility__("default")))
#   else
#     define LLVM_C_ABI
#   endif
# endif
#endif

This way we can independently control whether LLVM-C is static or dynamic.

[compnerd]

I don't think that we need to bind this to the LLVM's build style. That is, it should be safe to use a shared llvmc and a static LLVM.

[compnerd]

I think that this should remain part of the ABI.

[andrurogerz]

This one is weird. LLVMRemarkVersion is implemented in a different library here. I don't know what the correct solution is here, but the implementation is not linked into llvm so can't be exported by llvm. It feels like either the header doesn't belong here or the implementation should be moved so it does link into llvm.