[llvm] [gn] use action() instead of copy() for libcxx headers (PR #195948)

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darker --check --diff -r origin/main...HEAD llvm/utils/gn/build/sync_source_dir.py
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``````````diff
--- sync_source_dir.py	2026-05-05 21:45:33.000000 +0000
+++ sync_source_dir.py	2026-05-05 21:47:02.643733 +0000
@@ -17,18 +17,18 @@
         return f.read()
 
 
 def main():
     parser = argparse.ArgumentParser(
-        description=__doc__,
-        formatter_class=argparse.RawDescriptionHelpFormatter)
-    parser.add_argument("--stamp", required=True,
-                        help="name of a file whose mtime is updated on run")
+        description=__doc__, formatter_class=argparse.RawDescriptionHelpFormatter
+    )
+    parser.add_argument(
+        "--stamp", required=True, help="name of a file whose mtime is updated on run"
+    )
     parser.add_argument("--source-dir", required=True)
     parser.add_argument("--output-dir", required=True)
-    parser.add_argument("--except", dest="exceptions", action="append",
-                        default=[])
+    parser.add_argument("--except", dest="exceptions", action="append", default=[])
     parser.add_argument("rspfile")
     args = parser.parse_args()
 
     files = shlex.split(read(args.rspfile))
 

``````````

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https://github.com/llvm/llvm-project/pull/195948