Support LLVM 5.0

[ysbaddaden]

Adds support for LLVM 5.0 (rc2). They didn't introduce much breaking changes, for once.

Warning: builds on #4396 (drop LLVM 3.5 and 3.6 support) which should be merged beforehand, or merged directly with this pull request.

[ysbaddaden]

The LLVM5 release notes:
http://llvm.org/svn/llvm-project/llvm/branches/release_50/docs/ReleaseNotes.rst

[akzhan]

Just want to note that we must drop support for buggy LLVM <3.9. So please rebuild omnibuses and get LLVM 5 compatibility.

[sdogruyol]

LLVM 5.0 is just released http://releases.llvm.org/5.0.0/docs/ReleaseNotes.html 👍
LGTM?

[asterite]

CI failed. There's some bug in the LLVM bindings that I can't find... an LLVMContext is disposed but apparently the debug metadata associated with it was already disposed... but I don't know where...

[asterite]

In any case it's something that I think should only happen in compiler specs, and it has nothing to do with the LLVM version, so we can tackle that later at some point.

Should we merge this? All specs are passing, which is really good because they broke once brew install llvm changes to LLVM 5.0 :-)

[ysbaddaden]

I rebuilt on Circle and it passed this time. Random failures...

I'll be happy to merge. Omnibus should already be using LLVM 3.8 now, and if need be it may revert thus patch to reenable LLVM 3.5, but I doubt this is needed.

[Sija]

@RX14 what about #4396 mentioned in PR's description?