[clangd] Print qualifiers of out-of-line definitions in document outline

Summary: To improve the UX around navigating and searching through the results.

Reviewers: hokein

Reviewed By: hokein

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D66215

llvm-svn: 368842