[clangd] Expose the xref's incomplete flag to clangdServer API.

Summary: so that clangd C++ API users (via ClangdServer) can access it.

Reviewers: sammccall

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D70380