[clangd] Dont display <unknown> kinds in hover board
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.
Subscribers: ilya-biryukov, MaskRay, jkorous, arphaman, cfe-commits
Differential Revision: https://reviews.llvm.org/D72777