[clangd] Implement item kind for completion results

Summary: The patch implements the conversion method from CXCursorKind to clangd::CompletionItemKind.
Contributed by stanionascu!

Reviewers: cfe-commits, bkramer, krasimir

Reviewed By: krasimir

Tags: #clang-tools-extra

Differential Revision: https://reviews.llvm.org/D31853

llvm-svn: 299935