make a new llvm package (v6.0.0) (now with lld!)

[cosmicexplorer]

In pantsbuild/pants#5663, I talk about adding lld to the llvm tools we already provide. We were actually already doing this on osx because LLVM provides a binary package for osx include clang, lld, and a few other goodies, but for linux you have to individually check out a subdirectory in svn or download a separate tarball. We continue to download tarballs to build an LLVM release from source on linux, so this pr adds the lld source tarball and configures the build to enable building and installing lld. There is no functional change to the binary for osx -- the build-clang.sh script was renamed to build-llvm.sh and a few minor edits were made to the script.

For context on LLVM releases and building them from source in Linux, see the LLVM Download Page and Building LLVM with CMake.

I will be making a pants pr in a bit which shows how this would be consumed: the Clang BinaryTool will be removed in favor of an LLVM BinaryTool because this package no longer provides just clang. You can hold off on review for now if you want until that pr goes through review. The scripts worked for me on the first try on my linux and osx machines -- my attempt using the pantsbuild/centos6:latest docker container did run out of virtual memory, but I don't think that's going to be due to the build because it's worked on other linux environments and there would likely be some setting I could change to fix that.

[stuhood]

Thanks! Will wait to see the pantsbuild/pants review before reviewing this.