[clangd] Use llvm::errs() instead of outs() for errors

Summary: errs() is more appropriate for error messages in dexp and clangd-index-server.

Reviewers: sammccall

Reviewed By: sammccall

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D83827