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

[Joseph Huber via cfe-commits]

jhuber6 wrote:

> 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)

This code is pretty old, but we definitely should just be using `Unused` and `Unknown`. We should keep `SM_52` and `GFX1030` I'd say.

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