[clangd] Change line break behaviour for hoverinfo

`parseDocumentation` retains hard line breaks and removes soft line
breaks inside documentation comments.
Wether a line break is hard or soft is determined by the following rules
(some of which have been discussed in
https://github.com/clangd/clangd/issues/95):

Line breaks that are preceded by a punctuation are retained
Line breaks that are followed by "interesting characters" (e.g. Markdown
syntax, doxygen commands) are retained
All other line breaks are removed

Related issue: https://github.com/clangd/clangd/issues/95

Differential Revision: https://reviews.llvm.org/D76094