[clangd] Fix whitespace between chunks in markdown paragraphs.

Summary:
Old model: chunks are always separated by one space.
           This makes it impossible to render "Foo `bar`." correctly.

New model: chunks are separated by space if the left had trailing space, or
           the right had leading space, or space was explicitly requested.
           (Only leading/trailing space in plaintext chunks count, not code)

Reviewers: kadircet

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D79139