[clangd] Fuzzy match scorer
This will be used for rescoring code completion results based on partial
- we want to limit the number of code completion results returned to improve performance of global completion. The scorer will be used to rerank the results to return when the user has applied a filter.
Long-term use case:
- ranking of completion results from in-memory index
- merging of completion results from multiple sources (merging usually works best when done at the component-score level, rescoring the fuzzy-match quality avoids different backends needing to have comparable scores)
Subscribers: cfe-commits, mgorny
Differential Revision: https://reviews.llvm.org/D40060