Document the modifications to llvm and maybe upstream them

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).

The issue names no files, tests, or entry points. Start by inventorying the custom LLVM patches and comparing them with LLVM 12 APIs; done means documenting the modifications, identifying which patches can be upstreamed, and separating any remaining non-upstreamable work.

Written by the indexing model from the issue text.