[clangd] Show function documentation in signature help

Summary:
Previously, clangd was trying to show documentation for the active
parameter instead, which is wrong per LSP specification.

Moreover, the code path that attempts to get parameter comments never
succeds, because no attempt is made to parse function doc comment and
extract parameter-specific parts out of it. So we also remove the code
that claims to fetch parameter comments: it is not used anymore and is
incorrect.

Reviewers: hokein, ioeric, kadircet

Reviewed By: ioeric

Subscribers: MaskRay, jkorous, arphaman, cfe-commits

Differential Revision: https://reviews.llvm.org/D50726

llvm-svn: 340004