[clangd] Deduplicate clang-tidy diagnostic messages.

Summary:
Clang-tidy checks may emit duplicated messages (clang-tidy tool
deduplicate them in its custom diagnostic consumer), and we may show
multiple duplicated diagnostics in the UI, which is really bad.

This patch makes clangd do the deduplication, and revert the change
rL363889.

Reviewers: sammccall

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D64127

llvm-svn: 365204