[clangd] Added highlighting for structured bindings.
Summary: Structured bindings are in a BindingDecl. The decl the declRefExpr points to are the BindingDecls. So this adds an additional if statement in the addToken function to highlight them.
Reviewers: hokein, ilya-biryukov
Subscribers: MaskRay, jkorous, arphaman, kadircet, cfe-commits
Differential Revision: https://reviews.llvm.org/D66738