Re-add workaround for LLVM bug 11663

[glaubitz]

This re-adds the workaround for LLVM bug 11663 which is unfortunately still present in LLVM 6.0 (see: https://bugs.llvm.org/show_bug.cgi?id=11663). It was lost when compiler-rt was updated to LLVM 6.0.

[alexcrichton]

@glaubitz I can merge this here again, but it'd also be great to get this upstream! Our forks tend to turn into jungles that are very difficult to understand what's what, so "always just reset to upstream and go from there" tends to be our update strategy.

[glaubitz]

@alexcrichton Yes, I will talk to upstream and try to get it merged so this doesn't break again for the next LLVM bump.

Btw, is rust-llvm-release-6-0-0 going to be the default branch soon or should we also merge this into master? I'm a bit confused over which branches are the default ones.

[glaubitz]

Just as a heads-up: This patch has now been merged upstream, I was happy to be able to talk with LLVM folk at FOSDEM and convinced them we need this fix upstreamed:

https://reviews.llvm.org/D42902

[glaubitz]

This commit now got lost during the re-sync of compiler-rt from upstream in rust-lang/compiler-builtins@3454479.

I have no idea how to open a pull request to re-add the work-around because there is no branch I can open the PR against.

[infinity0]

@glaubitz This probably needs to be submitted to LLVM, since that's who hosts the compiler-rt project.

[glaubitz]

Already done:

https://reviews.llvm.org/D42902
https://reviews.llvm.org/D43146