Create safe LLVM bindings

[brson]

rustc's LLVM bindings are a bit of a hodgepodge. Some abstractions have been added on top of the C bindings, but not consistently. It would be nice to do some serious cleanup here so that rustc is using only safe, high-level bindings.

[emberian]

Visiting for triage. Still an issue. I'm not convinced we should continue using their C API, but it's incredibly inconvenient to have to wrap the C++ one manually. Their C++ API is slowly evolving under the C API, and the C API is frozen.

[steveklabnik]

I'm pulling a massive triage effort to get us ready for 1.0. As part of this, I'm moving stuff that's wishlist-like to the RFCs repo, as that's where major new things should get discussed/prioritized.

This issue has been moved to the RFCs repo: rust-lang/rfcs#644