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

[via cfe-commits]

tcottin wrote:

This patch only adapted the newline handling for `CommentFormat: Plaintext` to restore the newline behaviour for clients requesting Markdown without supporting Markdown newline rendering behaviour.

The `Markdown` and `Doxygen` options are new features introduced in clangd 22.
These features specify that the documentation comments are interpreted as Markdown or Doxygen respectively.
Therefore, Markdown is not escaped at all for these features.
They rely on clients to be able to render Markdown when they request Markdown content.

I think to get a "mix" of the new features and clients without Markdown rendering support, another configuration option could be introduced with a different patch.

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