Document the modifications to llvm and maybe upstream them

[KOLANICH]

Relying on a custom version of LLVM prevents proper packaging. It is inconvenient to have multiple versions of llvm on a machine. And it is especially inconvenient to build a custom llvm, since building of it takes long. I wonder if your patches can be upstreamed and built and shipped as a part of vanilla llvm, and if there are patches that cannot be upstreamed, if they can be put into a separate lib. Of course, a prereq for that is to port them all to the latest llvm (currently 12, but the differences (in API) from 11 are pretty small).