[llvm] [LIT] Add support for `%basename_s` to get base name of source file (PR #110993)

[Paul Kirth via llvm-commits]

https://github.com/ilovepi commented:

This seems mostly fine, but we should probably have a test under llvm/utils/lit/tests (https://github.com/llvm/llvm-project/tree/main/llvm/utils/lit/tests), rather than just using it in a single tablegen test. Would you mind moving the test there?

https://github.com/llvm/llvm-project/pull/110993