[clangd] Add support for type hierarchy (super types only for now)

Summary:
Patch by Nathan Ridge(@nridge)!

This is an LSP extension proposed here:
https://github.com/Microsoft/vscode-languageserver-node/pull/426

An example client implementation can be found here:
https://github.com/theia-ide/theia/pull/3802

Reviewers: kadircet, sammccall

Reviewed By: kadircet

Subscribers: jdoerfert, sammccall, cfe-commits, mgorny, dschaefer, simark, ilya-biryukov, ioeric, MaskRay, jkorous, arphaman, kadircet

Tags: #clang

Differential Revision: https://reviews.llvm.org/D56370

llvm-svn: 356445