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

[Fangrui Song via cfe-commits]

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@@ -356,13 +356,17 @@ class LLVM_LIBRARY_VISIBILITY Darwin : public AppleMachO {
   mutable bool TargetInitialized;
 
   enum DarwinPlatformKind {
+    // Platforms with associated *_DEPLOYMENT_TARGET environment variables.
     MacOS,
     IPhoneOS,
     TvOS,
     WatchOS,
     DriverKit,
     XROS,
-    LastDarwinPlatform = XROS
+    LastDarwinPlatform = XROS,
+
+    // Additional platforms.
+    Firmware,
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MaskRay wrote:

Adding `Firmware` after `LastDarwinPlatform` is pretty strange. Needs a detailed explanation

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