[clangd] Use fuzzy match to select top N index results.

Summary:
This makes performance slower but more predictable (it always processes
every symbol). We need to find ways to make this fast, possibly by precomputing
short queries or capping the number of scored results. But our current approach
is too naive.

It also no longer returns results in a "good" order. In fact it's pathological:
the top N results are ranked from worst to best. Indexes aren't responsible for
ranking and MergedIndex can't do a good job, so I'm pleased that this will make
any hidden assumptions we have more noticeable :-)

Reviewers: hokein

Subscribers: klimek, ilya-biryukov, cfe-commits

Differential Revision: https://reviews.llvm.org/D42060

llvm-svn: 322821