[clangd] Deduplicate refs from index for cross-file rename.

Summary:
If the index returns duplicated refs, it will trigger the assertion in
BuildRenameEdit (we expect the processing position is always larger the
the previous one, but it is not true if we have duplication), and also
breaks our heuristics.

This patch make the code robost enough to handle duplications, also
save some cost of redundnat llvm::sort.

Though clangd's index doesn't return duplications, our internal index
kythe will.

Reviewers: ilya-biryukov

Subscribers: MaskRay, jkorous, mgrang, arphaman, kadircet, usaxena95, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D71300

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