[clangd] Optionally add reflection for clangd-index-server

This was originally landed without the optional part and reverted later:

https://github.com/llvm/llvm-project/commit/8080ea4c4b8c456c72c617587cc32f174b3105c1

Reviewed By: kadircet

Differential Revision: https://reviews.llvm.org/D98404