nir: update fallthrough comments

clang doesn't support /* fallthrough */ so switch to fallthrough
attribute.

Reviewed-by: Kristian H. Kristensen <hoegsberg@google.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7747>

src/compiler/nir/nir_lower_io.c

@@ -613,6 +613,7 @@ nir_lower_io_block(nir_block *block,
          if (options->use_interpolated_input_intrinsics ||
              options->lower_interpolate_at)
             break;
+         FALLTHROUGH;
       default:
          /* We can't lower the io for this nir instrinsic, so skip it */
          continue;