[clangd] Only minimally escape text when rendering to markdown.

Summary:
Conservatively escaping everything is bad in coc.nvim which shows the markdown
to the user, and we have reports of it causing problems for other parsers.

Fixes https://github.com/clangd/clangd/issues/301

Reviewers: kadircet

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D75687