[clangd] Add `score` extension to workspace/symbol response.

The protocol doesn't really incorporate ranking.
As with code completion, most clients respect what the server sends, but
VSCode re-ranks items, with predictable results.
See https://github.com/clangd/vscode-clangd/issues/81

There's no filterText field so we may be unable to construct a good workaround.
But expose the score so we may be able to do this on the client in future.

Differential Revision: https://reviews.llvm.org/D88844