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

[Saleem Abdulrasool via cfe-commits]

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@@ -893,6 +901,8 @@ class Triple {
 
   bool isOSManagarm() const { return getOS() == Triple::Managarm; }
 
+  bool isOSFirmware() const { return getOS() == Triple::Firmware; }
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compnerd wrote:

This again applies without a vendor. Can we rename this to `isOSAppleFirmware`?

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