[clangd] Provide documentation as MarkupContent in signaturehelp

[clangd] Provide documentation as MarkupContent in signaturehelp

This unifies the behaviour we have in code completion item
documentations and signaturehelp. Providing better line wrapping and detection
of inline code blocks in comments to be renedered appropriately in markdown.

Differential Revision: https://reviews.llvm.org/D115442