Rollup merge of #47710 - alexcrichton:llvm-6-compat, r=nikomatsakis

First round of LLVM 6.0.0 compatibility

This includes a number of commits for the first round of upgrading to LLVM 6. There are still [lingering bugs](https://github.com/rust-lang/rust/issues/47683) but I believe all of this will nonetheless be necessary!

diff --cc src/librustc_llvm/ffi.rs
Simple merge

diff --cc src/librustc_trans/back/write.rs
Simple merge

diff --cc src/rustllvm/RustWrapper.cpp
Simple merge