Add llvm-project VMR source mapping

[tmds]

No description provided.

[premun]

This is weird, I can't seem to be able to initialize the repo locally even with this. So this is something to do with the LLVM repo files - maybe path length or something. The issue is the same as in the original PR and git doesn't provide much details just that it can't open some path...

I will try to initialize the repo from Linux

[premun]

I am still looking into some things before we merge this. Something is off with the LLVM diff we create to pull the repository in:

git --cached --ignore-space-change --directory src/llvm-project D:\tmp\llvm-project-4b825dc-30e9e6b.patch
error: git apply: failed to read: No such file or directory