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

[Khem Raj via cfe-commits]

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@@ -16,7 +16,7 @@ class StringRef;
 namespace clang {
 
 enum class OffloadArch {
-  UNUSED,
+  UNUSED_,
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kraj wrote:

@shiltian @efriedma-quic OK, I will rework it, thanks for feedback.

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