Don't discard the result of std::unique

After upgrading libc++, std::unique is marked [[nodiscard]], and it is
a build error to discard its result.

Cherry pick this commit:
https://github.com/llvm/llvm-project/commit/dd870f6929ee0b1dfb5fd000c7b826a1bd2d2571

Bug: b/175635923
Test: treehugger
Change-Id: I99fbe80b755f0ff4d9c897e91f2f15e334d5f1f6
1 file changed