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
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Closes #37609.
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**Test results:**
Everything is green 🎉
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| .. | ||
| build.rs | ||
| Cargo.toml | ||
| lib.rs | ||