[clangd] Render code complete documentation as plaintext/markdown.

Summary:
Structure is parsed from the raw comment using the existing heuristics used
for hover.

Reviewers: kadircet

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D79157