Triple::normalize: Set OS for 3-component triple with none as middle

If the middle component of a 3-component triple fails to parse as known
Arch/Vendor/OS/Env, it will fallback as Vendor. While for some cases,
we may wish to recognize it as OS, such as `arm64-none-elf`.

In this patch, we will set OS as `none`, if:
	1) Arch is found;
	2) Env is found;
	3) OS is not found and thus is set as empty;
	4) Vendor is not found while is set as "none",
we will swap Component[2] and Component[3].

Fixes: #89582.

Author: wzssyqa

llvm/lib/TargetParser/Triple.cpp

@@ -1149,6 +1149,13 @@ std::string Triple::normalize(StringRef Str) {
     }
   }
 
+  // For 3-component triples, the middle component is used to set Vendor;
+  // while if it is "none", we'd prefer to set OS.
+  // This is for some baremetal cases, such as "arm-none-elf".
+  if (Found[0] && !Found[1] && !Found[2] && Found[3] &&
+      Components[1].equals("none") && Components[2].empty())
+    std::swap(Components[1], Components[2]);
+
   // Replace empty components with "unknown" value.
   for (StringRef &C : Components)
     if (C.empty())