[clangd] Store index::SymbolKind in HoverInfo

Summary:
LSP's SymbolKind has some shortcomings when it comes to C++ types,
index::SymbolKind has more detailed info like Destructor, Parameter, MACRO etc.

We are planning to make use of that information in our new Hover response, and
it would be nice to display the Symbol type in full detail, rather than some
approximation.

Reviewers: sammccall

Subscribers: ilya-biryukov, MaskRay, jkorous, arphaman, usaxena95, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D70723