[llvm] [llvm-lit] Precommit Tests for implementing the `unset` command in lit internal shell (PR #104618)

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Harini0924 wrote:

That makes sense. Given that `env -u` is a standard Linux shell command and that `unset` isn’t widely used in the tests, the additional complexity in lit might not be justified. I’ll go ahead and close this PR.

https://github.com/llvm/llvm-project/pull/104618