[clangd] Simplify auto hover

Summary:
Use helper from clang. Also fixes some weird corner cases, e.g.
    auto (*foo)() = bar;

Reviewers: kadircet, hokein

Reviewed By: kadircet, hokein

Subscribers: ioeric, MaskRay, jkorous, arphaman, cfe-commits

Differential Revision: https://reviews.llvm.org/D53347

llvm-svn: 345128