[clangd] Add a hidden tweak to dump symbol under the cursor.

Summary:
This provides a convenient way to see the SymbolID/USR of the symbol, mainly
for debugging purpose.

Reviewers: ilya-biryukov

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D69517