Decouple LLVM from semantic passes

[Vici37]

Feature Request

As mentioned in this post, there is a requirement on LLVM when trying to include only the necessary parts for semantic processing. The local work around is to install LLVM 18 which removes the need for the llvm_ext.o shim, but for tooling that would like to expand on what the compiler itself brings, this would require all other crystal devs using said tools to also manually install LLVM 18+.

It was recommended here that I create an issue for this.

Thanks @ysbaddaden and @straight-shoota for the help so far!