[clangd] Fix DocumentOutline for concepts

Summary: Fixes https://github.com/clangd/clangd/issues/256

Reviewers: kbobyrev

Subscribers: ilya-biryukov, MaskRay, jkorous, arphaman, usaxena95, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D73056