[llvm-branch-commits] [lit] Make not still fail if the called process returns a signal (PR #174298)

[Aiden Grossman via llvm-branch-commits]

https://github.com/boomanaiden154 created https://github.com/llvm/llvm-project/pull/174298

This is the behavior of the main not binary that was not preserved in
the internal shell. Make it so that the builtin not command does
actually fail if we end up with a signal rather than just a non-zero
exit code.