[llvm] [llvm] annotate interfaces in llvm/CGData and llvm/CodeGen for DLL export (PR #140823)

[Saleem Abdulrasool via llvm-commits]

compnerd wrote:

> Why is this called `LLVM_ABI` (rather than `LLVM_API`)?

While it annotates the *API*, the declaration itself becomes part of the *ABI*. The annotations are to indicate what is the ABI boundary of the module to which it is applied to.

https://github.com/llvm/llvm-project/pull/140823