[SYCL] Fix -Werror=unused-but-set-variable warning with old gcc

[aelovikov-intel]

Follow-up for #18477.

[aelovikov-intel]

Wasn't caught in pre-commit of #18477 because that task was failing due to infrastructure issues with Jenkins job at the moment.

[bader]

Wasn't caught in pre-commit of #18477 because that task was failing due to infrastructure issues with Jenkins job at the moment.

The old gcc provides better diagnostics than the new one. 😮

[aelovikov-intel]

Wasn't caught in pre-commit of #18477 because that task was failing due to infrastructure issues with Jenkins job at the moment.

The old gcc provides better diagnostics than the new one. 😮

  constexpr auto x = ...;
  if constexpr (c0) {
     // no-x-uses
  } else if constexpr (c1) {
    // use-x
  } else if constexpr (c2) {
    // another-use-x
  } else if constexpr (c3) {
    // no-x-uses
  }

I'd say attribute shouldn't be required and new gcc is correct not to emit the diagnostic.