[clang] [Clang] Rename OffloadArch::UNUSED to UNUSED_ to avoid macro collisions (PR #174528)

[Eli Friedman via cfe-commits]

https://github.com/efriedma-quic commented:

Why is this enum using all-caps names in the first place?  It's against coding standard. (https://llvm.org/docs/CodingStandards.html#name-types-functions-variables-and-enumerators-properly)

https://github.com/llvm/llvm-project/pull/174528