[llvm] [lit] Add LIT_CURRENT_TESTCASE environment variable when running tests (PR #168762)

[Aiden Grossman via llvm-commits]

https://github.com/boomanaiden154 commented:

This seems like a reasonable enough feature to me for the stated use case.

It would be good if this had a test though, at least for the internal shell.

https://github.com/llvm/llvm-project/pull/168762