[clangd] Include the diagnostics's code when comparing diagnostics

Summary: This fixes https://github.com/clangd/clangd/issues/60

Reviewers: kadircet

Reviewed By: kadircet

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D63316

llvm-svn: 363889