[llvm] [llvm-(min-)tblgen] Avoid redundant source compilation (PR #114494)

[Michael Kruse via llvm-commits]

Meinersbur wrote:

@chapuni You already LGTM'ed this PR and did not revoke it after the edit, so I am assuming you are still OK with it (there are no copied files, but two new boilerplate ones that in the future may diverge when adding new features to only one of them). So I will push this at the end of this week. It would still be good if you explicitly OK's this change.

https://github.com/llvm/llvm-project/pull/114494