Nix Packaging

Hello. I've been sizing up the work and want to verify my conclusions. I'm going to state everything as fact and let errors be corrected.

The following binaries will be produced:

- lld
- clang/clang++
- create_llvm_prof

To create a full LLVM toolchain, these will be patched over a vanilla LLVM toolchain.

The llvm-propeller repo builds with a full llvm checkout as accompanying source. These sources are merged prior to build following the pattern here: https://lists.llvm.org/pipermail/llvm-dev/2019-September/135393.html

How does all of this look?

It's not clear to me what other libraries will need to be pulled along with the three binaries listed. While they will all build with static links, there's no sense in providing two `libllvm`s. I may need to change something to just output an entire llvm rather than patching two sets together. With a bit more familiarity with cmake and LLVM, I may be able to ask better questions, but I'm still just getting started with both of these code bases and Clang in general.