[clangd] Don't collect templated decls for builtin templates (#78466)

Builtin templates e.g. `__make_integer_seq`, `__type_pack_element` are
such that they don't have alias *Decls*.
[D133262](https://reviews.llvm.org/D133262) marked these as alias
templates, resulting in an attempt to collect their null "using" Decls
within our `TargetFinder`.

This fixes https://github.com/clangd/clangd/issues/1906.