[llvm] [llvm][Tablegen][llvm-tblgen] Added keyword #undef to llvm-tblgen (PR #69135)

[Mehdi Amini via llvm-commits]

https://github.com/joker-eph requested changes to this pull request.

I have some concerns right now because this would technically allow to include the same TableGen file multiple times, which is something we've been recently discussing of taking advantage of forbidding, and making the `#include` basically `#include_once`.

I would rather make sure we close to the door to multiple include of the same file before accidentally opening it!

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