Triple::normalize: Use none as OS for XX-none-ABI

When parsing a 3-component triple, after we determine Arch and Env,
if the middle component is "none", treat it as OS instead of Vendor.

Fixes: #89582.

Author: wzssyqa

clang/docs/Multilib.rst

@@ -188,9 +188,9 @@ For a more comprehensive example see
   - Dir: thumb/v6-m
     # List of one or more normalized command line options, as generated by Clang
     # from the command line options or from Mappings below.
-    # Here, if the flags are a superset of {target=thumbv6m-none-unknown-eabi}
+    # Here, if the flags are a superset of {target=thumbv6m-unknown-none-eabi}
     # then this multilib variant will be considered a match.
-    Flags: [--target=thumbv6m-none-unknown-eabi]
+    Flags: [--target=thumbv6m-unknown-none-eabi]
 
   # Similarly, a multilib variant targeting Arm v7-M with an FPU (floating
   # point unit).

llvm/lib/TargetParser/Triple.cpp

@@ -1151,6 +1151,12 @@ std::string Triple::normalize(StringRef Str) {
     }
   }
 
+  // If "none" is in the middle component in a three-component triple, treat it
+  // as the OS (Components[2]) instead of the vendor (Components[1]).
+  if (Found[0] && !Found[1] && !Found[2] && Found[3] &&
+      Components[1] == "none" && Components[2].empty())
+    std::swap(Components[1], Components[2]);
+
   // Replace empty components with "unknown" value.
   for (StringRef &C : Components)
     if (C.empty())