[llvm] [RFC][utils] Add script to update failed tests (PR #131637)

[Henrik G. Olsson via llvm-commits]

hnrklssn wrote:

> There was #108425, which added an `--update-tests` option to lit. Lit knows the correct build directory to use.
> 
> It was reverted, but I don't know why. @hnrklssn?

It was reverted because of a CI failure. After the fact, it seems like my patch may not have been the cause of the failure after all, but I never got around to re-landing it. I'll try to find some cycles and re-land it before the patch bitrots too much.

https://github.com/llvm/llvm-project/pull/131637