[clang] [Clang] Fix assertion "unsigned range includes negative?" in AnalyzeComparison during Sema of vector comparison with mismatched signed/unsigned types and __builtin_convertvector (PR #182627)

[Aaron Ballman via cfe-commits]

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AaronBallman wrote:

Please spell out the expected warnings

https://github.com/llvm/llvm-project/pull/182627