Comment # 3 on bug 1133254 from Aaron Puchert

LLVM is built in two stages: the first stage builds only Clang using GCC, the
second stage uses that Clang to compile the entire package. Clang has its own
form of LTO, but do you want to use that?

Then you'd have to build llvm-ar, llvm-ranlib, LLVMgold in addition to clang in
the first stage, and pass them into the second stage. I'm trying this out with
llvm7 in my home repo right now. However, I think there is probably not much
benefit to be gained, because LLVM has a lot of inline code in headers. In
addition, LLVM builds already take very long, and even just ThinLTO is going to
make that worse.