[llvm] [llvm][Tablegen][llvm-tblgen] Added keyword #undef to llvm-tblgen and fixed a small b… (PR #69135)

[Jon Roelofs via llvm-commits]

https://github.com/jroelofs requested changes to this pull request.

Please add test cases for the new behavior. You can find some examples under `llvm/test/TableGen`.

https://github.com/llvm/llvm-project/pull/69135