Travis: Add LLVM 5.0

[JohanEngelen]

LLVM 5.0 was just released.

[kinke]

There's still no prebuilt archive for Debian x64 (from LLVM directly), they gotta be kidding.

[JohanEngelen]

Ubuntu x64 is there, just the filename is following a different pattern :/

[JohanEngelen]

Wow, we have a conflicting cmdline option "dwarf-version" with LLVM 5.0, and it doesn't bug on my machine. :(
(I really think we should put all LLVM options behind -llvm like clang does, so we won't have these collisions and can remove the manual list of options to be removed from the help)

[JohanEngelen]

Perhaps the LLVM binaries are a strange build. The -dwarf-version option is not supposed to be in the LLVM libs that we link with...?

[kinke]

You're most likely building with LLD, right? LLD 5.0+ needs #2148.

[JohanEngelen]

Ah, yes it's building with LLD.

[JohanEngelen]

I've added -DLDC_WITH_LLD=OFF in a spot where it's easily spotted, reminding us to enable it when it works.