[clangd] Dont display `<unknown>` kinds in hover board

Summary:
Currently when hovering over an `auto` or `decltype` that resolve to a
builtin-type, clangd would display `<unknown>` as the kind of the symbol.

Drop that to make rendering nicer.

Reviewers: usaxena95

Subscribers: ilya-biryukov, MaskRay, jkorous, arphaman, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D72777

(cherry picked from commit b08e8353)