[clangd] Fetch documentation from the Index during signature help

Summary:
Sema can only be used for documentation in the current file, other doc
comments should be fetched from the index.

Reviewers: hokein, ioeric, kadircet

Reviewed By: hokein, kadircet

Subscribers: MaskRay, jkorous, mgrang, arphaman, cfe-commits

Differential Revision: https://reviews.llvm.org/D50727

llvm-svn: 340005