[llvm] [llvm] add documentation for public interface annotations (LLVM_ABI, etc) (PR #134710)

[Andrew Rogers via llvm-commits]

andrurogerz wrote:

+ @compnerd, @fsfod, @vvassilev

This write-up is based on my exercise of annotating most of the LLVM codebase, and I think it is worth merging it prior to merging more annotations.

https://github.com/llvm/llvm-project/pull/134710