[llvm-lit] Add Windows .cmd wrapper to make llvm-lit directly runnable (#155226)
On Linux/Mac, `llvm-lit` is configured with a shebang and made
executable so tools like LNT and the test-suite can invoke it directly.
On Windows the build only produces `llvm-lit.py` which cannot be used as
a standalone executable. This caused problems when running the LLVM
test-suite via LNT or buildbots.
This change introduces a new template file `llvm-lit.cmd.in` and updates
`llvm/utils/llvm-lit/CMakeLists.txt` so that a corresponding
`llvm-lit.cmd` is generated in the `build/bin` directory (for both
single-config and multi-config generators). The wrapper simply invokes
the configured Python interpreter on the adjacent `llvm-lit.py` and
propagates the exit code.
This ensures that `llvm-lit` can be used as a direct executable on
Windows just like on Linux without requiring external wrappers or
modifications in buildbot scripts or LNT.