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

When we parse 3-components triples, if the Arch and Env have been
parsed successfully, we have to make a choice between Vendor and
OS for the unrecoginzed component. Noramlly it is the middle one.

In the current code, Vendor is choosed, and then OS is fallbacked
to unknown. It is OK for most cases. But if the unrecoginzed
component is `none`, it is expected to be 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).

libcxx/utils/ci/run-buildbot

@@ -217,7 +217,7 @@ function test-armv7m-picolibc() {
         "${@}"
 
     ${NINJA} -vC "${BUILD_DIR}/compiler-rt" install
-    mv "${BUILD_DIR}/install/lib/armv7m-none-unknown-eabi"/* "${BUILD_DIR}/install/lib"
+    mv "${BUILD_DIR}/install/lib/armv7m-unknown-none-eabi"/* "${BUILD_DIR}/install/lib"
 
     check-runtimes
 }

llvm/lib/TargetParser/Triple.cpp

@@ -1151,6 +1151,13 @@ 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())