[clangd] Use ML Code completion ranking as default.

This makes code completion use a Decision Forest based ranking algorithm to rank
completion candidates. [Esitmated 6% accuracy boost]. This was
previously hidden behind the flag --ranking-model=decision_forest. This
patch makes it the default ranking algorithm.

Note: this is a generic model, not specialized for any particular
project. clangd does not collect or upload data to train code completion.

Also treat Keywords separately as they are not recorded by the training set generator.

Differential Revision: https://reviews.llvm.org/D96353