[clangd] Separate chunks with a space when rendering markdown

Summary:
This results in better rendering of resulting markdown.

Especially noticeable in coc.nvim that does not have a visible horizontal
spaces around inline code blocks. More details and a screenshot from
coc.nvim can be found in https://github.com/clangd/clangd/issues/95.

Reviewers: sammccall

Reviewed By: sammccall

Subscribers: MaskRay, jkorous, arphaman, kadircet, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D66086

llvm-svn: 368581