Incorporate update_*_test_checks.py into llvm-lit

[jroelofs]

Capturing the idea here from a discussion on Discord:

• It is annoying to have to specify tool paths to UTC scripts, but this is something llvm-lit has first-class knowledge of, so no more awkward --llc-binary/--clang
• llvm-lit knows how to do test discovery, parallel job dispatch, and gives a nice progress bar when you use -sv

[jroelofs]

cc @arsenm @nhaehnle @nikic