[clangd] Add index export to dexp

Summary: Add a command to dexp that exports index data in chosen format (e.g. YAML).

Reviewers: sammccall

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

Tags: #clang-tools-extra, #clang

Differential Revision: https://reviews.llvm.org/D77385