[clangd] Fix duplicate highlighting tokens appearing in initializer lists.

Summary: The RecursiveASTVisitor sometimes visits exprs in initializer lists twice. Added deduplication to prevent duplicate highlighting tokens from appearing. Done by sorting and a linear search.

Reviewers: hokein, sammccall, ilya-biryukov

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D64634

llvm-svn: 366070