[clangd] More sensible output for constructors/destructors in hover.

Summary:
Previously: both had type void() and return type void.
Now: neither have a type. Constructors return T, destructors return void.

Reviewers: hokein

Subscribers: merge_guards_bot, ilya-biryukov, MaskRay, jkorous, arphaman, kadircet, usaxena95, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D70317