Update LLVM

Upstream LLVM has changed slightly such that our PassWrapper.cpp no longer
comiles (travis errors). This updates the bundled LLVM to the latest nightly
which will hopefully fix the travis errors we're seeing.
auto merge of #12548 : alexcrichton/rust/up-llvm, r=huonw,brson

Upstream LLVM has changed slightly such that our PassWrapper.cpp no longer
comiles (travis errors). This updates the bundled LLVM to the latest nightly
which will hopefully fix the travis errors we're seeing.