[clang] [llvm] Reland "[Utils] Add new --update-tests flag to llvm-lit" (PR #153821)

[Nikita Popov via cfe-commits]

nikic wrote:

> It also adds support for auto-fixing tests failing based on the diff command.

This seems like an independent feature -- do it as a followup?

https://github.com/llvm/llvm-project/pull/153821