[clangd] Add a trained DecisionForest for code completion.

Replaces the dummy CodeCompletion model with a trained DecisionForest
model.
The features.json needs to be manually curated specifying the features
to be used. This is a one-time cost and does not change if the model
changes until we decide to add/remove features.

Differential Revision: https://reviews.llvm.org/D88071