Move LLVM bindings to their own crate

[brson]

Right now our LLVM bindings are tightly integrated into trans. Would be good for abstraction sake to fence LLVM off into its own crate. It would also make LLVM more available to other Rust projects - Rust is a good language for writing compilers.

cc #8274

[treeman]

Visiting for triage. Still relevant.

[huonw]

This is actually somewhat addressed, with the new rustc_llvm crate; I don't know the full details, but from a quick perusal it still has some Rust specific parts (e.g. the external library (source), and 1, 2, 3), thus AFAICT, it's not a fully generic abstraction.

However it may be enough to close this issue; thoughts, @brson?

[treeman]

My bad.

[steveklabnik]

I think this is about as addressed as it's going to get. We have the rustc_llvm crate for internal use, and people can use Cargo if they want to make better / nonrustc specific bindings.

@brson, if you disagree, feel free to re-open :)