[clangd] Ensure that we reply to each call exactly once. NFC (I think!)

Summary:
In debug builds, getting this wrong will trigger asserts.
In production builds, it will send an error reply if none was sent,
and drop redundant replies. (And log).

No tests because this is always a programming error.
(We did have some cases of this, but I fixed them with the new dispatcher).

Reviewers: ioeric

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

Differential Revision: https://reviews.llvm.org/D53399

llvm-svn: 345144