[clangd] Handle the new Using TemplateName.

Add supports in FindTarget and IncludeCleaner. This would
improve AST-based features on a tempalte which is found via a using
declaration. For example, go-to-def on `vect^or<int> v;` gives us the
location of `using std::vector`, which was not previously.

Base on https://reviews.llvm.org/D123127

Differential Revision: https://reviews.llvm.org/D123212