[clangd] Fix one testcase in XRefsTests.

Summary:
The test didn't test anything actually -- it used "[]" as annotation which should be
"[[]]".

This patch also fixes a bug in XRef where we may return duplicated refs.

Reviewers: ilya-biryukov

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D66349

llvm-svn: 369387