It’s still possible to build parts of the main llvm build (lld, clang etc) by symlinking them into llvm/tools.
We shouldn’t need this for the runtimes; they have their own story for how they’re built (which is complicated enough in other ways), and I don’t think anyone is symlinking then around.
Secondly, we’d need to set this in llvm/CMakeLists.txt too, to make it work automatically.
I can try to fix the patch and commandeer it.
Removed the changes to runtimes; the ways they are built is a separate story - afaik we mandate the monorepo layout there (and the symlinking setup isn't used there afaik).
Added a default setting in llvm/CMakeLists.txt, renamed to match LLVM_THIRD_PARTY_DIR. Made it a plain internal variable - it's not supposed to user settable in general (I think we're moving to mandate the monorepo layout overall).