[clang] [llvm] [Utils] Add new --update-tests flag to llvm-lit (PR #108425)

[Henrik G. Olsson via cfe-commits]

hnrklssn wrote:

Note that the first 4 commits are from https://github.com/llvm/llvm-project/pull/97369; only the last 2 are new. I'll merge that tomorrow when I have more time to be ready to revert in case something should happen.

https://github.com/llvm/llvm-project/pull/108425