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

[Paul Kirth via llvm-commits]

================
@@ -0,0 +1,5 @@
+# Basic test for substitutions.
+#
+# RUN: echo %basename_s  | FileCheck %s --check-prefix=BASENAME
+#
+# BASENAME: substitutions.py
----------------
ilovepi wrote:

For a file w/ a single check, I think `CHECK` is probably a better choice over a specific prefix. If you plan to follow up w/ more tests, then using the prefix makes sense though, so I'll leave that up to you to decide on the better approach.

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