[SYCL-MLIR][NFC] Fix warnings in `polygeist` (#12303)
- Replace deprecated member function cast-like calls with free
function calls
- Replace `startswith` member function call with `starts_with`
- Use `nodiscard` `ChangeResult` value returned by `setModifier`.
Signed-off-by: Victor Perez <victor.perez@codeplay.com>
