[clangd] Enable partial namespace matches for workspace symbols

This will enable queries like "clangd::" to find symbols under clangd
namespace, without requiring full "clang::clangd::" qualification.

Since Fuzzyfind performs the search under all scopes and only boosts the symbols
from relevant namespaces, we might get symbols from non-matching namespaces.
This patch chooses to drop those as they clearly do not match the query.

Fixes https://github.com/clangd/clangd/issues/550.

Differential Revision: https://reviews.llvm.org/D88814