[clangd] Simplify code of ClangdLSPServer::onCommand

Summary:
By inlining a complicated lambda into its single call-site.

Also ensure we call Reply() exactly once even if tweaks return both
ShowMessage and ApplyEdit effects.

Reviewers: hokein

Reviewed By: hokein

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D66343

llvm-svn: 369100