[clangd] Sort GoToDefinition results.

Summary:
GoToDefinition returns all declaration results (implicit/explicit) that are
in the same location, and the results are returned in arbitrary order.

Some LSP clients defaultly take the first result as the final result, which
might present a bad result (implicit decl) to users.

This patch ranks the result based on whether the declarations are
referenced explicitly/implicitly. We put explicit declarations first.

This also improves the "hover" (which just take the first result) feature
in some cases.

Reviewers: ilya-biryukov

Subscribers: kadircet, ioeric, MaskRay, jkorous, mgrang, arphaman, cfe-commits

Differential Revision: https://reviews.llvm.org/D50438

llvm-svn: 341463