[clangd] Show documentation in hover, and fetch docs from index if needed.

Summary:
I assume showing docs is going to be part of structured hover rendering, but
it's unclear whether that's going to make clangd 9 so this is low-hanging fruit.

(Also fixes a bug uncovered in FormattedString's plain text output: need blank
lines when text follows codeblocks)

Reviewers: kadircet

Subscribers: ilya-biryukov, MaskRay, jkorous, arphaman, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D64296

llvm-svn: 365522