[clangd] Fuzzy match scorer

Summary:
This will be used for rescoring code completion results based on partial
identifiers.
Short-term use:
  - 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)

Reviewers: ilya-biryukov

Subscribers: cfe-commits, mgorny

Differential Revision: https://reviews.llvm.org/D40060

llvm-svn: 319557