[clang] [llvm] [Utils] Adds support for diff based tests to lit's --update-tests (PR #154147)

[Henrik G. Olsson via cfe-commits]

hnrklssn wrote:

This is stacked on top of https://github.com/llvm/llvm-project/pull/153821

https://github.com/llvm/llvm-project/pull/154147