[llvm] [llvm] optionally install backend headers (PR #115837)

[Saleem Abdulrasool via llvm-commits]

https://github.com/compnerd commented:

I'm not sure how I feel about this change. I can see the value for out-of-tree clients. However, there are no ABI guarantees and there is no library versioning. As long as these are paired with a static library it _might_ be okay.

How do we ensure that the transitive closure is captured and nothing accidentally is left out?

https://github.com/llvm/llvm-project/pull/115837