Use a stable release of LLVM

[brson]

Requires #4259. Nominating. Do we want to be able to claim this for 1.0?

[pnkfelix]

(leaving to team mtg...)

[brson]

This doesn't necessarily mean upstreaming all our patches as long as Rust can be built with a stable release of LLVM.

[flaper87]

As a reminder taken from some comments in #4259, this specific issue is quite important for packaging. For instance, it won't be possible to upload a Rust package to fedora repos unless we start using stable llvm releases.

[flaper87]

FWIW, I gave it a try and it doesn't compile with stable llvm (version 3.4). When my queue of things to do gets shorter, I could give this a try (unless someone wants to start playing with this right away)

[flaper87]

I believe this is done. We've been using upstream llvm 3.3 and 3.4 (Now upgrading to 3.5 #13513) for our travis builds. I'll leave the close call to @alexcrichton who has been working heavily on this area.

[alexcrichton]

Hurray! Yes, we will be fully compatible with 3.5 by the time it comes out.