The `ghc-toolchain` overhaul will eventually replace all this stuff with
something much more cleaned up, but I think it is still worth making
this sort of cleanup in the meantime so other untanglings and dead code
cleaning can procede.

I was able to delete a fair amount of dead code doing this too.

`LLVMTarget_CPP` is renamed to / merged with `LLVMTarget` because it
wasn't actually turned into a valid CPP identifier. (Original to
1345c7cc, actually.)

Progress on #23966

Co-Authored-By: Sylvain Henry <hsyl20@gmail.com>