[llvm] [Dexter] Add ability to rewrite scripts to fill-in unknown values (PR #202799)

[Jeremy Morse via llvm-commits]

https://github.com/jmorse approved this pull request.

I think the final "explain why we're using diff in the tests" bit has been missed, LGTM once that's addressed.

https://github.com/llvm/llvm-project/pull/202799