[clang] [llvm] [Triple] Make an target triple "os" for firmware (PR #176272)

[Fangrui Song via cfe-commits]

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@@ -3118,9 +3139,7 @@ bool Darwin::isAlignedAllocationUnavailable() const {
   case WatchOS: // Earlier than 4.0.
     OS = llvm::Triple::WatchOS;
     break;
-  case XROS: // Always available.
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MaskRay wrote:

These apple platform simplification can be extracted as a separate patch.

https://github.com/llvm/llvm-project/pull/176272