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

[Chris B via llvm-commits]

llvm-beanz wrote:

I'm not really a fan of complicating our build by supporting this configuration, but that isn't a strongly held opinion.

Looping in some other contributors with build expertise: @petrhosek & @compnerd.

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