[libc++] Update the <version> header in-place from generate_feature_test_macro_components

This simplifies the workflow for adding new feature-test macros for
contributors. Previously, they would have to move the generated <version>
header from a temporary directory to libc++'s include directory by hand.
This makes the behavior for the <version> header consistent with what's
done for the tests and the documentation.

GitOrigin-RevId: 647fb6b37488080efd8dd5e5a40d21e926b6e726
diff --git a/docs/DesignDocs/FeatureTestMacros.rst b/docs/DesignDocs/FeatureTestMacros.rst
index 2fbba65..644eb4a 100644
--- a/docs/DesignDocs/FeatureTestMacros.rst
+++ b/docs/DesignDocs/FeatureTestMacros.rst
@@ -39,7 +39,5 @@
 update feature test macros in libc++.
 
 Whenever a feature test macro is added or changed, the table should be updated
-and the script should be re-ran. The script will clobber the existing test files
-and the documentation and it will generate a new `<version>` header as a
-temporary file. The generated `<version>` header should be merged with the
-existing one.
+and the script should be re-ran. The script will clobber the existing test files,
+the documentation and the `<version>` header.