0777c757a6
LLVM 4.0 Upgrade Since nobody has done this yet, I decided to get things started: **Todo:** * [x] push the relevant commits to `rust-lang/llvm` and `rust-lang/compiler-rt` * [x] cleanup `.gitmodules` * [x] Verify if there are any other commits from `rust-lang/llvm` which need backporting * [x] Investigate / fix debuginfo ("`<optimized out>`") failures * [x] Use correct emscripten version in docker image --- Closes #37609. --- **Test results:** Everything is green 🎉