[llvm] [llvm-readobj] Add a reusable LLVMReadobjDriver library (PR #222917)

[Fangrui Song via llvm-commits]

https://github.com/MaskRay requested changes to this pull request.

I don't think this should land, at least in its current form.

The library doesn't add a capability. `GENERATE_DRIVER` already gives `llvm_readobj_main(int, char **, const ToolContext &)`, and with LLVM_TOOL_LLVM_DRIVER_BUILD=ON the sources are already compiled into the obj.llvm-readobj object library that the multicall llvm binary links. This PR renames a directory and adds an install/export rule for the same objects; nothing changes for an embedder except the CMake target name.

I also don't see general agreement on the Discourse on this direction. There are other people, like me, confused by the direction.

https://github.com/llvm/llvm-project/pull/222917