Prepare for LLVM 18, link to local repo for scripts and patches

[alan-j-hu]

This references the scripts and patches for my fork of opam-source-archive, just so I can see if things work on CI... If this is accepted, they will need to be changed to point to ocaml/opam-source-archive.

Context: ocaml/opam-source-archives#40

[alan-j-hu]

I'm getting cascading errors because of this:

Error in conf-llvm-static.18: error 46: Package is flagged "conf" but has source, install or remove instructions
Error in conf-llvm-static.18: No package source directory provided.
Error in conf-llvm-shared.18: error 46: Package is flagged "conf" but has source, install or remove instructions
Error in conf-llvm-shared.18: No package source directory provided.

This is despite the fact that this all worked on my computer testing locally, and conf-llvm.17 and prior all have build fields and were accepted. However, today when I run opam lint on conf-llvm.17, I get this error. Is this something new?

[alan-j-hu]

Okay, I misunderstood the CI error message.