[llvm] [LIT] Rename substitution `%basename_s` to `%{s:basename}` (PR #111062)

[Alexander Richardson via llvm-commits]

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@@ -535,7 +535,6 @@ TestRunner.py:
  %{fs-tmp-root}          root component of file system paths pointing to the test's temporary directory
  %{fs-sep}               file system path separator
  %t                      temporary file name unique to the test
- %basename_s             The last path component of %s
  %basename_t             The last path component of %t but without the ``.tmp`` extension
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arichardson wrote:

```suggestion
 %basename_t             The last path component of %t but without the ``.tmp`` extension (deprecated, use ``%{t:stem}`` instead)
```
I wonder if we should also deprecate this inconsistent one?

https://github.com/llvm/llvm-project/pull/111062