[clangd] add an extension field to LSP to transfer the diagnostic's category

This patch adds a 'category' extension field to the LSP diagnostic that's sent
by Clangd. This extension is always on by default.

Differential Revision: https://reviews.llvm.org/D50571


git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@339738 91177308-0d34-0410-b5e6-96231b3b80d8
11 files changed