[clangd] Index UsingDecls

Summary:
D58340 enables indexing of USRs, this makes sure test in clangd are
aligned with the change

Reviewers: hokein

Subscribers: ilya-biryukov, ioeric, MaskRay, jkorous, arphaman, jdoerfert, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D58341

llvm-svn: 354879