[llvm] [llvm] add LLVM_FRIEND_ABI macro for friend function decls (PR #136595)

[Andrew Rogers via llvm-commits]

andrurogerz wrote:

@compnerd this change is an alternative to using the `__attribute__` style annotations for visibility in https://github.com/llvm/llvm-project/pull/135995. Even though it introduces a new macro, I think it is a better solution because it is less invasive and ensures we only introduce export annotations to `friend` functions where needed: on Windows.

There are only a couple of dozen total `friend` function declarations throughout LLVM, so I don't think the additional `LLVM_FRIEND_ABI` macro will be a maintenance burden.

https://github.com/llvm/llvm-project/pull/136595