[arch-dev-public] LLVM 6 and splitting up {clang, lld, lldb, compiler-rt} into separate PKGBUILDs

I pushed llvm 6.0.0-1 to [staging] without the rest of the packages mentioned in the subject. I am planning to add them back as individual packages in the following days. This separation will bring faster builds when one of these components needs patching. It also allows building clang with shared libraries and merging the extra-tools package into it (thus fixing https://bugs.archlinux.org/tas k/50865). Once everything is in [staging], I'll create a TODO list for rebuilds as usual.