[clang-tools-extra] [clangd] Use plaintext newline handling for escaped markdown hover style (PR #185197)

[Marvin Ewald via cfe-commits]

mphe wrote:

`CommentFormat: Plaintext` now looks as expected but `Markdown` and `Doxygen` still retain line breaks.

https://github.com/llvm/llvm-project/pull/185197