[llvm] [clang-tools-extra] [MLGO] Upstream the corpus extraction tooling (PR #72319)

[Mircea Trofin via cfe-commits]

mtrofin wrote:

> > Would it be also possible to remove the dependency on [Abseil](https://github.com/abseil/abseil-py)? None of the existing scripts in LLVM use it and I don't think we should be introducing this dependency. It looks like Abseil is only used for flag parsing, logging and testing; those should be straightforward to replace with standard libraries like `argparse`, `logging` or `unittest`.
> 
> Yes. My plan was to remove the dependency on abseil as well. My plan was to get this landed with all the infrastructure setup and the code basically just directly copied and then remove the abseil dependency in a follow-up patch so that the different pieces get reviewed appropriately.

Ah, if you can drop the abseil dependency, the dependency problem for tests goes away. May be worth doing the abseil dropping bit in this patch, too?

https://github.com/llvm/llvm-project/pull/72319