[clangd] Don't set the Underlying bit on targets of UsingDecls.

With this patch, we don't treat `using ns::X` as a first-class declaration like `using Z = ns::Y`, reference to X that goes through this using-decl is considered a direct reference (without the Underlying bit).

Fix the workaround in https://reviews.llvm.org/D87225 and https://reviews.llvm.org/D74054.

Reviewed By: sammccall

Differential Revision: https://reviews.llvm.org/D88472