[clangd][Hover] Change arrow in return type back to →

Summary:
Currently 🡺 is used in hover response to represent return types, but it
is not widely available. Changing this back to original to support more clients.

Reviewers: sammccall

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D73336

(cherry picked from commit 1b996faa)