[clangd] NFC: Use TopN instead of std::priority_queue

Quality.cpp defines a structure for convenient storage of Top N items,
it should be used instead of the `std::priority_queue` with slightly
obscure semantics.

This patch does not affect functionality.

Reviewed By: sammccall

Differential Revision: https://reviews.llvm.org/D51676

llvm-svn: 341544