[clang] [HLSL] Warn on duplicate is_rov attribute; remove unnecessary parentheses (PR #107973)

[Justin Bogner via cfe-commits]

https://github.com/bogner approved this pull request.

It's a little awkward to do all three of these things together in one change, but it all looks simple and correct.

https://github.com/llvm/llvm-project/pull/107973