[clangd] Filter completion results by fuzzy-matching identifiers.

Summary:
This allows us to limit the number of results we return and still allow them
to be surfaced by refining a query (D39852).

The initial algorithm is very conservative - it accepts a completion if the
filter is any case-insensitive sub-sequence. It does not attempt to rank items
based on match quality.

Reviewers: ilya-biryukov

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D39882

llvm-svn: 319552