[clangd] Add Random Forest runtime for code completion.

Summary:
[WIP]
- Proposes a json format for representing Random Forest model.
- Proposes a way to test the generated runtime using a test model.

TODO:
- Add generated source code snippet for easier review.
- Fix unused label warning.
- Figure out required using declarations for CATEGORICAL columns from Features.json.
- Necessary Google3 internal modifications for blaze before landing.
- Add documentation for format of the model.
- Document more.

Subscribers: mgorny, ilya-biryukov, MaskRay, jkorous, arphaman, kadircet, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D83814