[CodingStandard] Require Unix line endings for all files (#161228)

Require all files to use Unix line endings, formalizing an already
followed convention.

Author: jurahul

llvm/docs/CodingStandards.rst

@@ -1790,6 +1790,12 @@ would help to avoid running into a "dangling else" situation.
       markAsIgnored(D);
   }
 
+Use Unix line endings for files
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Use Unix line endings for all files. CRLF line endings are allowed as an
+exception for test files that intend to test CRLF handling or when the file
+format requires it (like ``.bat`` or ``.rc`` files).
 
 See Also
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