[clangd] Drop returntype/type when hovering over type-ish names

Summary:
Some names, e.g. constructor/destructor/conversions, already contain
the type info, no need to duplicate them in the hoverinfo.

Fixes https://github.com/clangd/clangd/issues/252

Reviewers: sammccall, ilya-biryukov

Subscribers: MaskRay, jkorous, arphaman, usaxena95, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D73110

(cherry picked from commit 1fbb1d6d)