[clangd] Add C++20 concepts support to findExplicitReferences() and semantic highlighting

Summary: Fixes https://github.com/clangd/clangd/issues/259

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D73124