[clangd] Introduce intermediate representation of formatted text

Summary: That can render to markdown or plain text. Used for findHover requests.

Reviewers: malaperle, sammccall, kadircet

Reviewed By: sammccall

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D58547

llvm-svn: 360151