Try to fix a periodic windows build failure due to broken rwildcard macro
Upgrade LLVM
Start building a multithreaded rust
auto merge of #6713 : alexcrichton/rust/llvm-upgrades, r=catamorphism

This includes some of @brson's patches, plus one of mine. A multithreaded-llvm is necessary for running `rusti` tests in parallel, and in general is a good idea for running `rustc` in parallel.